From GoG's facebook page:
"Awesome game arriving for adventure-game fans soon! 90%+ Metacritic rating and lots of people haven't even heard of it! Hopefully that'll change."

Well, a quick search yielded only one adventure game with a score above 90 which isn't from lucas arts. So it seems rather obvious, but I might have missed something.

Now thats got me really excited as the only adventure games i can see with a 90%+ score are Lucasarts.
Oh and The Longest Journey.

metacritic is weird and doesn't usually rate old games.

Yeah but the Longest Journey is fairly well known and still available today via places such as Steam.

If it's a game not many people have heard of I doubt it will be a Lucasarts game.
After doing a quick search on Metacritic my guess is The Longest Journey.

Maybe Deus Ex or The Longest journy?
Metacritic Search

there's a very short list of 90+ games on metacritic
http://www.metacritic.com/games/pc/scores/
there are only 2 games on that list that are visibly adventure games that have a 90+ rating
SO IT LOOKS LIKE LUCASARTS IS COMING TO GOG!
Either Grim Fandango or Day Of the Tentacle.
edit:
also, it's important to note, that GOG's categorization is kind of unusual in my opinion.
if you click on adventure in the catalogue, you get only point and clickers, while there are other games on there that are adventure games in a more modern sense, they don't fall under the same category.

Many other game genres have adventure elements to them so it could still be something unexpected.

Well, the fact that it said many people hadn't heard of it is a bit weird. Surely LA games are well known anyway, and TLJ isn't exactly an unknown. More likely to be the latter than the former though.

Very exiting news, whichever game it turns out to be. There's a high probability that I'll have to buy it immediately after release.

In what circles? Certainly it is among GOG patrons.
And who really gives a crap about Metacritic? All it does is tell you what proportion of the game review industry (and it is an industry) gushed over a particular game. It doesn't even list most of the great old adventure games.

boy. longest journey is one i've been wanting to try for ages.... but i don't think you could say it's unknown. Its pretty famous. Same goes for lucasarts games.
So it must be Lula 3D.
(but considering metacritic filters Chaos Theory and Lego Star Wars as adventure games, i wouldn't be surprised if it's in a different category...)

Both any Lucasarts game and The Longest Journey must certainly be considered well-known among gamers. I'd bet it's something else entirely.
Syberia for instance?