Both scenarios are supported by the various factory and reader methods.
Besides uncompressed WARC files, GZip compressed files are also supportedThe ARC/WARC factory and reader methods support both compressed and uncompressed files in varying combinations.
Basicly you can create a reader for compressed or uncompressed files, but the factory also includes an auto-detection mechanism which can determine the correct reader on a per file basis.
GZip compression is only supported on ARC/WARC files where each record is compressed individually and concatenated into one file and not the case where the whole ARC/WARC file and all it's records are GZip'ed as a whole. The later mostly because this makes random access to individual record highly ineffective.