A week ago or so, I noticed Aquanox 2 can't be found in the game catalogue any more.
It still shows in my bought games lists (I downloaded a backup copy just in case it disappears from there as well), but the link to the game desc. is gone too.

I never saw any announcement or comment about it being taken down.
In fact it was on X-mas sale less than a month ago...

Anyone knows what happened?

The Aquanox titles were only available during GOG's Nordic In-N-Out sale. Nordic pulled its games from the catalog a few months ago, due to differences about regional pricing. Apparently GOG made a deal during Christmas to offer them for a limited time within a special sale offer. Therefore the name of the sale "In-N-Out". When the sale was over, the Nordic titles were removed from the catalog again, but that doesn't affect the games in your game shelf. What you've bought, is yours and won't vanish.

I see...

Thanks for the explanation.

Wierd. I still see it listed in store.

That's because GOG and Nordic reached an agreement and Nordic brought their games back to GOG. All of Nordic's games that were removed from the store are available again.