Very good idea. I completely agree. If GOG would concentrate on improving their website's functionality, that should be one of the things being dealt with immediately.

As strange as it may sound, I would really like the ability to go both ways: show only games not already owned, or show only games owned.

I guess there is no need to explain the first filter, a lot of people have been asking for it for as long as I can remember…
The second option would be to browse discounted games I already own, so during sales I can easily spot games I would like to recommend to friends.

GOG.com like all those affacted by "big company syndrome" only listens to fans/players/customers/ if they say what they already think.

The "we'll pass the suggestion on to...." (or what ever) basically means "we'll just forget all about it".

The fact that people have been asking for "hide owned games" for years proves this (personal opinion).

Hmm, only reason I see for showing games already owned is if one wants to buy gifts. Now I am unsure whether we can buy gifts of games we do NOT own already as I only ever bought gifts of games I had in my library.

Yepp, I also would like such a filter.

Yes.

If you use Ublock Origin this seems to work :

www.gog.com##.product-tile:has(> .product-tile__content:has(> .product-tile__cover:has(> .product-tile__labels--in-library)))

To be clear all this does is hide any game that would have the in library image on them.

No idea if its a good idea as is or whatever just thought I'd give it a shot. I'd guess its going to slow things down a bunch.

It obviously won't do antthing for the soundtracks/dlc you don't own and in cases where you own something but with a different ID you might miss the hint that GOG saying you own the base game gives.

Edited: to add the domain

This is seriously awesome. Why didn't I think of misusing Ublock that way? :-) I changed that to

gog.com##.product-tile:has(> .product-tile__content:has(> .product-tile__cover:has(> .product-tile__labels--in-library)))

so that the filter will only be applied to the domain gog.com but otherwise your filter works like a charm. Thank you for sharing.

Sorry for that, I copied it directly from the element picker box rather than the My Filters area. The picker shows the generic but when you commit the change it adds the domain to it.

Berfeoot essentials solves this. It can be found hereish.

edit

As a matter of interest, how slow is it?

I was kinda just saying that to make people consider the possibility if they noticed issues on the site as I hadn't tested it.
I'm kinda highly tolerant of slow down so I've never tried to quantify the effect a given rule has.
When I was testing it the "In Library" titles show for a second or two before vanishing not sure if that shows the time it takes to process the page or if it has an effect beyond that. So its probably a non issue unless you're using something like my old netbook that starts to overheat just thinking about opening a browser.

The reason I ask is because that's a triple-join. If those are three different tables then it will be a significant drain on whatever is performing the joins (the server, I presume), which would be quite a load if everyone did it. Which may explain why Gog hasn't implemented it, because it would crash the site. (Stealing other site servers' processing power would not bother them, but customers would have to subtract the baud cost, as well.)