diff --git a/stdlib/zlib/0/gzip_writer.rbs b/stdlib/zlib/0/gzip_writer.rbs
index 9dfc47c8c..4f6cc0836 100644
--- a/stdlib/zlib/0/gzip_writer.rbs
+++ b/stdlib/zlib/0/gzip_writer.rbs
@@ -38,7 +38,8 @@ module Zlib
     # returns a GzipWriter object associated with that file.  Further details of
     # this method are found in Zlib::GzipWriter.new and Zlib::GzipFile.wrap.
     #
-    def self.open: (String filename) { (instance gz) -> void } -> instance
+    def self.open: (String filename, ?Integer level, ?Integer strategy, **untyped opts) -> instance
+                 | (String filename, ?Integer level, ?Integer strategy, **untyped opts) { (instance gz) -> void } -> nil
 
     # <!--
     #   rdoc-file=ext/zlib/zlib.c
@@ -64,7 +65,7 @@ module Zlib
     # `flush` is same as in Zlib::Deflate#deflate.  `Zlib::SYNC_FLUSH` is used if
     # `flush` is omitted.  It is no use giving flush `Zlib::NO_FLUSH`.
     #
-    def flush: (?Integer flush) -> String
+    def flush: (?Integer flush) -> self
 
     # <!--
     #   rdoc-file=ext/zlib/zlib.c
@@ -83,7 +84,7 @@ module Zlib
     # compression started. Setting a value of 0 indicates no time stamp is
     # available.
     #
-    def mtime=: (path | IO file_name) -> Time
+    def mtime=: (Time | Integer) -> void
 
     # <!--
     #   rdoc-file=ext/zlib/zlib.c
diff --git a/test/stdlib/zlib/GzipWriter_test.rb b/test/stdlib/zlib/GzipWriter_test.rb
new file mode 100644
index 000000000..1bfda5c5f
--- /dev/null
+++ b/test/stdlib/zlib/GzipWriter_test.rb
@@ -0,0 +1,114 @@
+require_relative "../test_helper"
+require "zlib"
+require "tempfile"
+
+class ZlibGzipWriterSingletonTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "zlib"
+  testing "singleton(::Zlib::GzipWriter)"
+
+  def test_open
+    Dir.mktmpdir do |tmpdir|
+      path = File.join(tmpdir, "test.gz")
+      result = assert_send_type "(String) -> Zlib::GzipWriter", Zlib::GzipWriter, :open, path
+      result.close
+
+      assert_send_type "(String) { (Zlib::GzipWriter) -> void } -> nil",
+                       Zlib::GzipWriter, :open, path do |gz| end
+      assert_send_type "(String, Integer, Integer) { (Zlib::GzipWriter) -> void } -> nil",
+                       Zlib::GzipWriter, :open, path, Zlib::DEFAULT_COMPRESSION, Zlib::DEFAULT_STRATEGY do |gz| end
+      assert_send_type "(String, Integer, Integer, encoding: Encoding) { (Zlib::GzipWriter) -> void } -> nil",
+                       Zlib::GzipWriter, :open, path, Zlib::DEFAULT_COMPRESSION, Zlib::DEFAULT_STRATEGY, encoding: Encoding::UTF_8 do |gz| end
+
+    end
+  end
+
+  def test_new
+    assert_send_type "(StringIO) -> Zlib::GzipWriter",
+                     Zlib::GzipWriter, :new, StringIO.new
+    assert_send_type "(StringIO, Integer) -> Zlib::GzipWriter",
+                     Zlib::GzipWriter, :new, StringIO.new, Zlib::DEFAULT_COMPRESSION
+    assert_send_type "(StringIO, Integer, Integer) -> Zlib::GzipWriter",
+                     Zlib::GzipWriter, :new, StringIO.new, Zlib::DEFAULT_COMPRESSION, Zlib::DEFAULT_STRATEGY
+    assert_send_type "(StringIO, Integer, Integer, encoding: Encoding) -> Zlib::GzipWriter",
+                     Zlib::GzipWriter, :new, StringIO.new, Zlib::DEFAULT_COMPRESSION, Zlib::DEFAULT_STRATEGY, encoding: Encoding::UTF_8
+  end
+end
+
+class ZlibGzipWriterInstanceTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "zlib"
+  testing "::Zlib::GzipWriter"
+
+  def test_lshift
+    assert_send_type "(String) -> Zlib::GzipWriter",
+                     Zlib::GzipWriter.new(StringIO.new), :<<, "hello"
+  end
+
+  def test_comment=
+    assert_send_type "(String) -> void",
+                     Zlib::GzipWriter.new(StringIO.new), :comment=, "hello"
+  end
+
+  def test_flush
+    assert_send_type "() -> Zlib::GzipWriter",
+                     Zlib::GzipWriter.new(StringIO.new), :flush
+    assert_send_type "(Integer) -> Zlib::GzipWriter",
+                     Zlib::GzipWriter.new(StringIO.new), :flush, Zlib::SYNC_FLUSH
+  end
+
+  def test_mtime=
+    assert_send_type "(Time) -> void",
+                     Zlib::GzipWriter.new(StringIO.new), :mtime=, Time.now
+    assert_send_type "(Integer) -> void",
+                     Zlib::GzipWriter.new(StringIO.new), :mtime=, 0
+  end
+
+  def test_orig_name=
+    assert_send_type "(String) -> void",
+                     Zlib::GzipWriter.new(StringIO.new), :orig_name=, "hello"
+  end
+
+  def test_pos
+    assert_send_type "() -> Integer",
+                     Zlib::GzipWriter.new(StringIO.new), :pos
+  end
+
+  def test_print
+    assert_send_type "(String) -> nil",
+                     Zlib::GzipWriter.new(StringIO.new), :print, "hello"
+  end
+
+  def test_printf
+    assert_send_type "(String) -> nil",
+                     Zlib::GzipWriter.new(StringIO.new), :printf, "hello"
+    assert_send_type "(String, Integer) -> nil",
+                     Zlib::GzipWriter.new(StringIO.new), :printf, "%d", 1
+  end
+
+  def test_putc
+    assert_send_type "(String) -> String",
+                     Zlib::GzipWriter.new(StringIO.new), :putc, "h"
+    assert_send_type "(Integer) -> Integer",
+                     Zlib::GzipWriter.new(StringIO.new), :putc, "h".ord
+  end
+
+  def test_puts
+    assert_send_type "() -> nil",
+                     Zlib::GzipWriter.new(StringIO.new), :puts
+    assert_send_type "(String) -> nil",
+                     Zlib::GzipWriter.new(StringIO.new), :puts, "hello"
+  end
+
+  def test_tell
+    assert_send_type "() -> Integer",
+                     Zlib::GzipWriter.new(StringIO.new), :tell
+  end
+
+  def test_write
+    assert_send_type "(String) -> Integer",
+                     Zlib::GzipWriter.new(StringIO.new), :write, "hello"
+  end
+end