ZFR: You already have them. GOG is not telling you "buy the games somewhere else first"
Since other stores wouldn't have the same functionality but GOG would, GOG would basically be telling you "You know, you should first buy your games from the other stores, and only the leftovers from GOG because unlike the other stores, here you can track what you already have elsewhere.".
GOG would basically be making it easier for people to buy primarily and first from other stores.
Is this same lack of feature preventing you from buying from other stores, like Steam, Origin, UPlay etc.? If not, why not? They don't tell you either if you already have the game on some other store or service, including consoles.
ZFR: I come to the GOG page during sale. See a game and think "This might be interesting" and check it out and maybe buy it. If this game is hidden on page 5 I often don't bother going there (It's like Google. Nobody checks page 3).
It's easier to spot a game you might be potentially interested in if there is less clutter of unwanted games.
And how about other services like Steam, Origin, Humble Bundle etc.? Are you reluctant to buy anything from them because you can't easily check if you already have the game on some other service?
I presume you buy from those other services as well, as otherwise such feature would be mostly useless to you.
trusteft: First of all I don't care if any other stores have the hide option or not. I want to buy the games from GOG.
So you don't buy games from other stores or services then?
Since different people had a different idea what exactly you were requesting, which was it then?
a) You want GOG to hide games that you already own
in the GOG service itself? Currently they mark those games with the "in your account" icon (just like Steam seems to do), but not actually hide the games.
b) You want GOG to hide games that you also own on any other services or systems (I presume you mean manually marking games as "owned" and hiding them, as there is no meaningful way for GOG to track what you own on Steam, Origin, UPlay, retail CDs, Windows Store, PS4, XBox One etc.).
The first one GOG is tracking for you even if they don't actually hide such games from the lists or the main page.
The second one, it is not in GOG's best interests to offer such a feature. A feature to make it easier for people to NOT to buy games from GOG? Yeah, makes sense, almost. That must be why no other store has AND WILL NOT implement such a feature, ever, neverevereverEVAH!
trusteft: I am glad you know better than I know what I buy or not and why. There were times when I was just too bored to go through the list of the games on sale because many of them I already own.
I just skip those games which are marked as owned, and check the games which don't have that icon. I don't have to try to remember which I own on GOG already or not.