I've been looking for publishing HTML5 games using gzip but I couldn't figure out how to do it with Itch.io, so I've done some research and came across this Issue #394 from April 14th 2016 :
https://github.com/itchio/itch.io/issues/394
Is it not possible yet? I've put a game using gzip files and I get errors that the webserver does not provide the headers needed.