Is there something in the itch.io platform that requires or recommends people who label their game as GPL(3) to put a link to the source code somewhere? Because I'm noticing a few games claim they're licensed as GPL3, but I couldn't find a link to their source code. As of time of writing:
https://door-jam-studios.itch.io/choleric-vision
https://scriptangames.itch.io/harkanoid
https://briac.itch.io/tubular-craze
They are quite clearly labelled as GPL3 games if you click on "more information" and they don't appear to be all that difficult to find if you use code-gpl3 in the url. I haven't clicked the download links, but it's not obvious from the content that the source code is included in the download links.
If they don't want to release their source code, that's fine, but then it's not really a GPL game.