Fix Rewindable stream Length and add GZip Reader tests

src/SharpCompress/IO/RewindableStream.cs

@@ -84,11 +84,11 @@ public override void Flush()
             throw new NotSupportedException();
         }
 
-        public override long Length => throw new NotSupportedException();
+        public override long Length => stream.Length;
 
         public override long Position
         {
-            get { return stream.Position + bufferStream.Position - bufferStream.Length; }
+            get => stream.Position + bufferStream.Position - bufferStream.Length;
             set
             {
                 if (!isRewound)

src/SharpCompress/SharpCompress.csproj

@@ -2,9 +2,9 @@
   <PropertyGroup>
     <AssemblyTitle>SharpCompress - Pure C# Decompression/Compression</AssemblyTitle>
     <NeutralLanguage>en-US</NeutralLanguage>
-    <VersionPrefix>0.27.0</VersionPrefix>
-    <AssemblyVersion>0.27.0</AssemblyVersion>
-    <FileVersion>0.27.0</FileVersion>
+    <VersionPrefix>0.27.1</VersionPrefix>
+    <AssemblyVersion>0.27.1</AssemblyVersion>
+    <FileVersion>0.27.1</FileVersion>
     <Authors>Adam Hathcock</Authors>
     <TargetFrameworks>netstandard2.0;netstandard2.1;netcoreapp3.1;net5.0</TargetFrameworks>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>

tests/SharpCompress.Test/GZip/GZipReaderTests.cs

@@ -0,0 +1,37 @@
+using System.IO;
+using SharpCompress.Common;
+using SharpCompress.IO;
+using Xunit;
+
+namespace SharpCompress.Test.GZip
+{
+    public class GZipReaderTests : ReaderTests
+    {
+        public GZipReaderTests()
+        {
+            UseExtensionInsteadOfNameToVerify = true;
+        }
+
+        [Fact]
+        public void GZip_Reader_Generic()
+        {
+            Read("Tar.tar.gz", CompressionType.GZip);
+        }
+
+
+        [Fact]
+        public void GZip_Reader_Generic2()
+        {
+            //read only as GZip itme
+            using (Stream stream = File.OpenRead(Path.Combine(TEST_ARCHIVES_PATH, "Tar.tar.gz")))
+            using (var reader = SharpCompress.Readers.GZip.GZipReader.Open(new RewindableStream(stream)))
+            {
+                while (reader.MoveToNextEntry()) // Crash here
+                {
+                    Assert.NotEqual(0, reader.Entry.Size);
+                    Assert.NotEqual(0, reader.Entry.Crc);
+                }
+            }
+        }
+    }
+}