Is there any way to hide games already in your library from the gog store page? So often the banner "You may like these products" is often just full of games I already own and have played.

The only place you can hide owned games is the All Games page, where you can check "Hide all owned products" in the sidebar.

Using a web browser and an add-on like uBlock Origin, you can filter out various things. But a lot of the time, this leaves behind a blank space, or messes with things that load dynamically, so it's less than great overall.