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; }