diff --git a/core/gc.rbs b/core/gc.rbs
index 457fd0b468..bcafac6ede 100644
--- a/core/gc.rbs
+++ b/core/gc.rbs
@@ -7,6 +7,16 @@
 # You may obtain information about the operation of the GC through GC::Profiler.
 #
 module GC
+  # <!-- rdoc-file=gc.c -->
+  # internal constants
+  #
+  INTERNAL_CONSTANTS: Hash[Symbol, Integer | bool]
+
+  # <!-- rdoc-file=gc.c -->
+  # GC build options
+  #
+  OPTS: Array[String]
+
   # <!--
   #   rdoc-file=gc.rb
   #   - GC.count -> Integer
@@ -70,7 +80,7 @@ module GC
   # are not guaranteed to be future-compatible, and may be ignored if the
   # underlying implementation does not support them.
   #
-  def self.start: (?immediate_sweep: boolish immediate_sweep, ?immediate_mark: boolish immediate_mark, ?full_mark: boolish full_mark) -> nil
+  def self.start: (?immediate_sweep: boolish, ?immediate_mark: boolish, ?full_mark: boolish) -> nil
 
   # <!--
   #   rdoc-file=gc.rb
@@ -156,8 +166,8 @@ module GC
   #
   # This method is only expected to work on CRuby.
   #
-  def self.stat: (?::Hash[Symbol, Integer] arg0) -> ::Hash[Symbol, Integer]
-               | (?Symbol arg0) -> Integer
+  def self.stat: (?Hash[Symbol, Integer]? hash) -> Hash[Symbol, Integer]
+               | (Symbol key) -> Integer
 
   # <!--
   #   rdoc-file=gc.rb
@@ -165,7 +175,7 @@ module GC
   # -->
   # Returns current status of GC stress mode.
   #
-  def self.stress: () -> (Integer | TrueClass | FalseClass)
+  def self.stress: () -> (Integer | bool)
 
   # <!--
   #   rdoc-file=gc.rb
@@ -183,7 +193,8 @@ module GC
   #     0x02:: no immediate sweep
   #     0x04:: full mark after malloc/calloc/realloc
   #
-  def self.stress=: (Integer | TrueClass | FalseClass flag) -> (Integer | TrueClass | FalseClass)
+  def self.stress=: (Integer flag) -> Integer
+                  | (bool flag) -> bool
 
   # <!--
   #   rdoc-file=gc.rb
@@ -210,7 +221,11 @@ module GC
   #
   #     GC.respond_to?(:compact)
   #
-  def self.compact: () -> ::Hash[:considered | :moved, Hash[Symbol | Integer, Integer]]
+  def self.compact: () -> compact_info
+
+  # The type that `GC.compact` and related functions can return.
+  #
+  type compact_info = Hash[:considered | :moved |:moved_up | :moved_down, Hash[Symbol, Integer]]
 
   # <!--
   #   rdoc-file=gc.rb
@@ -227,7 +242,7 @@ module GC
   # a full GC.  If any object contains a reference to a T_MOVED object, that
   # object should be pushed on the mark stack, and will make a SEGV.
   #
-  def self.verify_compaction_references: () -> ::Hash[:considered | :moved, Hash[Symbol | Integer, Integer]]
+  def self.verify_compaction_references: (?toward: :empty | untyped, ?double_heap: boolish, ?expand_heap: boolish) -> compact_info
 
   # <!--
   #   rdoc-file=gc.c
@@ -251,8 +266,8 @@ module GC
   # If the optional argument, hash, is given, it is overwritten and returned. This
   # is intended to avoid probe effect.
   #
-  def self.latest_gc_info: () -> ::Hash[::Symbol, untyped]
-                         | [K] (?Hash[K, untyped] hash) -> ::Hash[::Symbol | K, untyped]
+  def self.latest_gc_info: (?nil) -> Hash[Symbol, untyped]
+                         | (Hash[Symbol, untyped] hash) -> Hash[Symbol, untyped]
                          | (Symbol key) -> untyped
 
   # <!--
diff --git a/test/stdlib/GC_test.rb b/test/stdlib/GC_test.rb
index df631eca92..d34933c8a3 100644
--- a/test/stdlib/GC_test.rb
+++ b/test/stdlib/GC_test.rb
@@ -1,65 +1,240 @@
-require_relative "test_helper"
+require_relative 'test_helper'
 
-class GCTest < StdlibTest
-  target GC
+class GCSingletonTest < Test::Unit::TestCase
+  include TestHelper
 
-  include GC
+  testing "singleton(::GC)"
 
-  def test_garbage_collect
-    garbage_collect
-    garbage_collect(full_mark: true)
-    garbage_collect(full_mark: false)
-    garbage_collect(immediate_mark: true)
-    garbage_collect(immediate_mark: false)
-    garbage_collect(immediate_sweep: true)
-    garbage_collect(immediate_sweep: false)
+  def test_INTERNAL_CONSTANTS
+    assert_const_type 'Hash[Symbol, Integer | bool]',
+                      'GC::INTERNAL_CONSTANTS'
+  end
+
+  def test_OPTS
+    assert_const_type 'Array[String]',
+                      'GC::OPTS'
+  end
+
+  def test_count
+    assert_send_type  '() -> Integer',
+                      GC, :count
+  end
+
+  def test_disable
+    was_disabled = GC.disable
+
+    assert_send_type  '() -> bool',
+                      GC, :disable
+  ensure
+    GC.enable unless was_disabled
+  end
+
+  def test_enable
+    was_enabled = GC.enable
+
+    assert_send_type  '() -> bool',
+                      GC, :enable
+  ensure
+    GC.disable unless was_enabled
   end
 
   def test_start
-    GC.start
-    GC.start(full_mark: true)
-    GC.start(full_mark: false)
-    GC.start(immediate_mark: true)
-    GC.start(immediate_mark: false)
-    GC.start(immediate_sweep: true)
-    GC.start(immediate_sweep: false)
+    assert_send_type  '() -> nil',
+                      GC, :start
+
+    [true, Object.new, nil].each do |immediate_sweep|
+      [true, Object.new, nil].each do |immediate_mark|
+        [true, Object.new, nil].each do |full_mark|
+          assert_send_type  '(immediate_sweep: boolish, immediate_mark: boolish, full_mark: boolish) -> nil',
+                            GC, :start, immediate_sweep: immediate_sweep, immediate_mark: immediate_mark, full_mark: full_mark
+        end
+      end
+    end
+  end
+
+  def test_stat
+    assert_send_type  '() -> Hash[Symbol, Integer]',
+                      GC, :stat
+    assert_send_type  '(Hash[Symbol, Integer]) -> Hash[Symbol, Integer]',
+                      GC, :stat, {}
+    assert_send_type  '(nil) -> Hash[Symbol, Integer]',
+                      GC, :stat, nil
+    assert_send_type  '(Symbol) -> Integer',
+                      GC, :stat, :count
+  end
+
+  def test_stress_and_stress=
+    old_stress = GC.stress
+
+    assert_send_type  '() -> (Integer | bool)',
+                      GC, :stress
+    assert_send_type  '(Integer) -> Integer',
+                      GC, :stress=, 0
+    assert_send_type  '() -> (Integer | bool)',
+                      GC, :stress
+    [true, false].each do |bool|
+      assert_send_type  '(bool) -> bool',
+                        GC, :stress=, bool
+    end
+  ensure
+    GC.stress = old_stress
+  end
+
+  def test_total_time
+    assert_send_type  '() -> Integer',
+                      GC, :total_time
   end
 
   def test_compact
-    GC.compact
+    assert_send_type  '() -> GC::compact_info',
+                      GC, :compact
   end
 
   def test_verify_compaction_references
-    GC.verify_compaction_references
+    assert_send_type  '() -> GC::compact_info',
+                      GC, :verify_compaction_references
   end
 
   def test_verify_internal_consistency
-    GC.verify_internal_consistency
+    assert_send_type  '() -> nil',
+                      GC, :verify_internal_consistency
   end
 
   def test_latest_gc_info
-    GC.latest_gc_info
-    GC.latest_gc_info({})
-    GC.latest_gc_info(:state)
+    assert_send_type  '() -> Hash[Symbol, untyped]',
+                      GC, :latest_gc_info
+    assert_send_type  '(nil) -> Hash[Symbol, untyped]',
+                      GC, :latest_gc_info, nil
+    assert_send_type  '(Hash[Symbol, untyped]) -> Hash[Symbol, untyped]',
+                      GC, :latest_gc_info, {}
+
+    assert_send_type  '(Symbol) -> untyped',
+                      GC, :latest_gc_info, :major_by
+  end
+
+  def test_auto_compact
+    assert_send_type  '() -> bool',
+                      GC, :auto_compact
+  end
+
+  def test_auto_compact=
+    old = GC.auto_compact
+
+    [true, nil, Object.new, 1r].each do |obj|
+      assert_send_type  '[T] (T) -> T',
+                        GC, :auto_compact=, obj
+    end
+  ensure
+    GC.auto_compact = old
+  end
+
+  def test_latest_compact_info
+    assert_send_type  '() -> GC::compact_info',
+                      GC, :latest_compact_info
   end
 
-  def test_set_stress
-    GC.stress = 0
-    GC.stress = true
-    GC.stress = false
+  def test_measure_total_time
+    assert_send_type  '() -> bool',
+                      GC, :measure_total_time
+  end
+
+  def test_measure_total_time=
+    old = GC.measure_total_time
+
+    [true, nil, Object.new, 1r].each do |obj|
+      assert_send_type  '[T] (T) -> T',
+                        GC, :measure_total_time=, obj
+    end
+  ensure
+    GC.measure_total_time = old
   end
 end
 
+class GCIncludeTest < Test::Unit::TestCase
+  include TypeAssertions
 
-class GCSingletonTest < Test::Unit::TestCase
-  include TestHelper
+  testing '::GC'
 
-  testing "singleton(::GC)"
+  class Foo
+    extend GC
+  end
+
+  def test_garbage_collect
+    assert_send_type  '() -> nil',
+                      Foo, :garbage_collect
+
+    [true, Object.new, nil].each do |immediate_sweep|
+      [true, Object.new, nil].each do |immediate_mark|
+        [true, Object.new, nil].each do |full_mark|
+          assert_send_type  '(immediate_sweep: boolish, immediate_mark: boolish, full_mark: boolish) -> nil',
+                            Foo, :garbage_collect, immediate_sweep: immediate_sweep, immediate_mark: immediate_mark, full_mark: full_mark
+        end
+      end
+    end
+  end
+end
+
+class GC_ProfilerSingletonTest < Test::Unit::TestCase
+  include TypeAssertions
+
+  testing 'singleton(::GC::Profiler)'
+
+  def test_clear
+    assert_send_type  '() -> nil',
+                      GC::Profiler, :clear
+  end
+
+  def test_enabled?
+    assert_send_type  '() -> bool',
+                      GC::Profiler, :enabled?
+  end
+
+  def test_enable_and_disable
+    was_enabled = GC::Profiler.enabled?
+
+    assert_send_type  '() -> nil',
+                      GC::Profiler, :enable
+    assert_send_type  '() -> nil',
+                      GC::Profiler, :disable
+  ensure
+    GC::Profiler.enable if was_enabled
+  end
+
+  def test_raw_data
+    was_enabled = GC::Profiler.enabled?
+    GC::Profiler.disable
+
+    assert_send_type  '() -> nil',
+                      GC::Profiler, :raw_data
+
+    GC::Profiler.enable
+    GC.start
+    assert_send_type  '() -> Array[Hash[Symbol, untyped]]',
+                      GC::Profiler, :raw_data
+  ensure
+    GC::Profiler.enable if was_enabled
+  end
+
+  def test_report
+    old_stdout = $stdout
+    $stdout = Object.new.tap { def _1.write(*) end }
+
+    assert_send_type  '() -> nil',
+                      GC::Profiler, :report
+
+    assert_send_type  '(_Writer) -> nil',
+                      GC::Profiler, :report, Writer.new
+  ensure
+    $stdout = old_stdout
+  end
+
+  def test_result
+    assert_send_type  '() -> String',
+                      GC::Profiler, :result
+  end
 
   def test_total_time
-    assert_send_type(
-      "() -> Integer",
-      GC, :total_time
-    )
+    assert_send_type  '() -> Float',
+                      GC::Profiler, :total_time
   end
 end