Fix GZip extraction for non-seekable streams

- Modified GZipFilePart to only access stream.Position when stream.CanSeek is true
- Modified GZipArchiveEntry.OpenEntryStream to check CanSeek before accessing Position
- Added test case GZip_Archive_NonSeekableStream to verify non-seekable stream support
- All existing tests pass

Co-authored-by: adamhathcock <527620+adamhathcock@users.noreply.github.com>

src/SharpCompress/Archives/GZip/GZipArchiveEntry.cs

@@ -15,9 +15,10 @@ public class GZipArchiveEntry : GZipEntry, IArchiveEntry
     {
         //this is to reset the stream to be read multiple times
         var part = (GZipFilePart)Parts.Single();
-        if (part.GetRawStream().Position != part.EntryStartPosition)
+        var rawStream = part.GetRawStream();
+        if (rawStream.CanSeek && rawStream.Position != part.EntryStartPosition)
         {
-            part.GetRawStream().Position = part.EntryStartPosition;
+            rawStream.Position = part.EntryStartPosition;
         }
         return Parts.Single().GetCompressedStream().NotNull();
     }

src/SharpCompress/Common/GZip/GZipFilePart.cs

@@ -24,8 +24,12 @@ internal sealed class GZipFilePart : FilePart
             stream.Position = stream.Length - 8;
             ReadTrailer();
             stream.Position = position;
+            EntryStartPosition = position;
+        }
+        else
+        {
+            EntryStartPosition = 0;
         }
-        EntryStartPosition = stream.Position;
     }
 
     internal long EntryStartPosition { get; }

tests/SharpCompress.Test/GZip/GZipArchiveTests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.IO;
 using System.Linq;
 using SharpCompress.Archives;
@@ -124,4 +125,60 @@ public class GZipArchiveTests : ArchiveTests
         using var archive = GZipArchive.Open(stream);
         Assert.Equal(archive.Type, ArchiveType.GZip);
     }
+
+    [Fact]
+    public void GZip_Archive_NonSeekableStream()
+    {
+        // Test that GZip extraction works with non-seekable streams (like HttpBaseStream)
+        using var fileStream = File.OpenRead(Path.Combine(TEST_ARCHIVES_PATH, "Tar.tar.gz"));
+        var buffer = new MemoryStream();
+        fileStream.CopyTo(buffer);
+        buffer.Position = 0;
+
+        // Create a non-seekable wrapper around the MemoryStream
+        using var nonSeekableStream = new NonSeekableStream(buffer);
+        using var reader = SharpCompress.Readers.GZip.GZipReader.Open(nonSeekableStream);
+
+        // Verify we can move to the first entry and read it without exceptions
+        Assert.True(reader.MoveToNextEntry());
+        Assert.NotNull(reader.Entry);
+
+        // Extract and verify the entry can be read
+        using var outputStream = new MemoryStream();
+        reader.WriteEntryTo(outputStream);
+
+        Assert.True(outputStream.Length > 0);
+    }
+
+    // Helper class to simulate a non-seekable stream like HttpBaseStream
+    private class NonSeekableStream : Stream
+    {
+        private readonly Stream _baseStream;
+
+        public NonSeekableStream(Stream baseStream) => _baseStream = baseStream;
+
+        public override bool CanRead => _baseStream.CanRead;
+        public override bool CanSeek => false; // Simulate non-seekable stream
+        public override bool CanWrite => false;
+        public override long Length => throw new NotSupportedException();
+
+        public override long Position
+        {
+            get => throw new NotSupportedException();
+            set => throw new NotSupportedException();
+        }
+
+        public override void Flush() => _baseStream.Flush();
+
+        public override int Read(byte[] buffer, int offset, int count) =>
+            _baseStream.Read(buffer, offset, count);
+
+        public override long Seek(long offset, SeekOrigin origin) =>
+            throw new NotSupportedException();
+
+        public override void SetLength(long value) => throw new NotSupportedException();
+
+        public override void Write(byte[] buffer, int offset, int count) =>
+            throw new NotSupportedException();
+    }
 }