two years ago and this is still a problem..
Viewing post in Is Itch.io still not providing Gzip support?
We added support for gziping html5 content a while ago, sorry this topic wasn't updated. It's white-listed to certain file types though, since many game engines will apply their own gzipping to binary asset files. If you have a particular file type/game you want me to investigate you can share a link.
Thanks