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.

At the bottom of this very page, there is 'directory' which has several links to only show games with various licenses.  :)

Technically under the GPL3 the person releasing a compiled binary can require that you contact them and (for free or a fee) receive a copy of the current version of the source code via physical mail and/or digitally via a private download link.

It is good/common practice for GPL'd software to have an online repository link accessible to anyone without having to ask permission from the developers first.  However LEGALLY speaking they only have to make it available 'upon request' for anyone that asks.  I suggest contacting the devs of these projects directly in addition to the forum post you've already made here.

GPL software itself is a bit of a thorny issue and also has mandatory requirements like putting a header in EVERY source code file to identify it as being GPL'd code.