Don't get your hopes up for a solution from GOG any time soon.

However, awesome community members have come up with solutions for all sorts of problems with the site, one of which is applicable to you: Adalia Fundamentals includes (among a LOT of other things) an option to hide your owned games in the store catalogue and among the promos on the front page. Another is Barefoot Essentials that has several other convenience functions/options.

Not exactly what you were asking for, if I understand you correctly, but it may be a slight improvement from the current situation.