Wireshark will transparently decompress a gzip pcap file. pcapgo.NewReader should check if the first couple of bytes are 0x1f 0x8b, and if so wrap the io.Reader with a gzip.NewReader.

This will allow pcapgo to open compressed pcap files, and would be consistent with how wireshark's wiretap library does it https://github.com/wireshark/wireshark/blob/90e1232672446e79e75e5c6877310a49f23ae228/wiretap/file_wrappers.c#L575

Happy to send a pull request.