diff --git a/.gitignore b/.gitignore
index 8baa9b4..3a63e6c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,3 +9,5 @@
 
 # rspec failure tracking
 *.rspec_status
+
+/build_gem/
diff --git a/spec/gem_server_conformance_spec.rb b/spec/gem_server_conformance_spec.rb
index f7c2869..a8bc69d 100644
--- a/spec/gem_server_conformance_spec.rb
+++ b/spec/gem_server_conformance_spec.rb
@@ -70,24 +70,43 @@
       io.binmode
       package = Gem::Package.new(io)
       actual = []
+
+      presence = proc do |x|
+        x if x && !x.empty?
+      end
+
       dump_tar = proc do |tar_io, into = actual|
         Gem::Package::TarReader.new(tar_io) do |gem|
           gem.each do |entry|
             body = entry.read
-            body = Zlib.gunzip(body) if entry.full_name.end_with?(".gz")
+            extra = nil
+            if entry.full_name.end_with?(".gz")
+              extra = Zlib::GzipReader.wrap(StringIO.new(body)) do |gz|
+                magic, compression_method, flags, mtime, compression, os_id = body.unpack("H4ccVcC")
+                {
+                  magic: magic, compression_method: compression_method, flags: flags, mtime: mtime,
+                  compression: compression, os_id: os_id, comment: presence[gz.comment], crc: gz.crc,
+                  orig_name: presence[gz.orig_name]
+                }.compact
+              end
+              body = Zlib.gunzip(body)
+            end
             body = dump_tar[StringIO.new(body), []] if entry.full_name.end_with?(".tar.gz")
 
             into << {
               header: entry.header.instance_variables.to_h do |ivar|
                         [ivar.to_s.tr("@", "").to_sym, entry.header.instance_variable_get(ivar)]
                       end,
-              body: body
-            }
+              body: body,
+              extra: extra
+            }.compact
           end
         end
         into
       end
       package.gem.with_read_io(&dump_tar)
+      expected_gz_extra = { compression: 2, compression_method: 8, crc: 0, flags: 0, magic: "1f8b", mtime: 0,
+                            os_id: 3 }
       expect(actual).to eq(
         [{ body: <<~YAML,
           --- !ruby/object:Gem::Specification
@@ -142,7 +161,8 @@
                      typeflag: "0",
                      uid: 0,
                      uname: "wheel",
-                     version: 0 } },
+                     version: 0 },
+           extra: expected_gz_extra },
          { body: [],
            header: { checksum: 5834,
                      devmajor: 0,
@@ -160,7 +180,8 @@
                      typeflag: "0",
                      uid: 0,
                      uname: "wheel",
-                     version: 0 } },
+                     version: 0 },
+           extra: expected_gz_extra },
          { body: <<~YAML,
            ---
            SHA256:
@@ -186,7 +207,8 @@
                      typeflag: "0",
                      uid: 0,
                      uname: "wheel",
-                     version: 0 } }]
+                     version: 0 },
+           extra: expected_gz_extra }]
       )
     end
 
diff --git a/spec/support/request_helpers.rb b/spec/support/request_helpers.rb
index 7e5a0d4..e486d30 100644
--- a/spec/support/request_helpers.rb
+++ b/spec/support/request_helpers.rb
@@ -23,6 +23,34 @@ def encode_with(coder)
     end
   end
 
+  module PackageGzipToConsistentOS
+    class IOWrapper < SimpleDelegator
+      def write(str)
+        str[9] = "\x03".b if str.size == 10 && str.start_with?("\x1f\x8b".b)
+        super
+      end
+    end
+
+    def gzip_to(io, &blk)
+      super(IOWrapper.new(io), &blk)
+    end
+  end
+
+  ::Gem::Dependency.class_eval do
+    if Gem::Dependency.method_defined?(:to_yaml_properties)
+      prepend(
+        Module.new do
+          def to_yaml_properties
+            expected = %i[@name @requirement @type @prerelease @version_requirements]
+            actual = super
+
+            (expected & actual) + (actual - expected)
+          end
+        end
+      )
+    end
+  end
+
   def build_gem(name, version, platform: nil)
     spec = Gem::Specification.new do |s|
       s.name = name
@@ -40,10 +68,17 @@ def build_gem(name, version, platform: nil)
     package = Gem::Package.new(StringIO.new.binmode)
     package.build_time = Time.utc(1970)
     package.spec = spec
+    package.singleton_class.prepend(PackageGzipToConsistentOS)
     package.gem.singleton_class.send(:define_method, :path) { "" }
 
     package.build
 
+    if ENV["DUMP_BUILD_GEM"]
+      tmp = "build_gem/#{RUBY_ENGINE}/rubygems-#{Gem::VERSION}/#{@time}"
+      FileUtils.mkdir_p(tmp)
+      File.binwrite("#{tmp}/#{spec.full_name}.gem", package.gem.io.string)
+    end
+
     MockGem.new(
       name: name,
       version: spec.version,