Hello, I would like to know whether Fez will return to GoG, since her absence looks strange against the background that she is still being sold in Steam?

No, i don't know.

It left on 2018-07-25, per request from the developer.

I don't exactly mean to slander, but unfortunately the developers of Fez, Polytronic suffered from a case of Tumblr syndrome.

Their lead developer, Phil Fish had an especially bad case of Auteur syndrome. Skin as thick as 0 ply paper. So he was full of himself and couldn't take critical reception of any kind that wasn't tongue bathing.

So to cut a long story short, a now former PR person for GOG said some things, and was fired. Unfortunately, nobody at Polytronic has thought to consider that time changes and that the person responsible is no longer here.

Fez is really unique and beautiful platformer with fun gameplay. The game was removed from the store due to it toxic developer Phil Fish who quarreled with the entire gaming industry. But who knows how things are now and who owns the rights to the game? You need to ask the publisher...