I know it's still quite recent, but the price is already well within the range of GOG's regular pricing. (it can be had in the "Gothic Universe" 3 pack for $20, or in the "RPG Fantasy Action Pack" for $15 - $20). It's also developed by the same people who did the first and second entries in the series, and the same publisher as well.
So, can we expect to see Gothic 3 on GOG in the future? (Near future, hopefully)

For contractual reasons, GOG doesn't talk about what games could be on the way or have no chance of coming. The best way you can help particular games get onto GOG is to vote for them on the wishlist; as the GOG staff have stated, games that have a lot of support theoretically have a higher chance of being added one day.

I've been voting on things on the wishlist like mad lately, but the issue is that everyone seems to vote for the games that already have thousands of votes, and not bother with anything else.

That's because those items are the ones on the list that are most visible, and because they are in the highest demand. Monkey Island 1, for instance, is renowned as a fantastic game, but good luck finding a copy nowadays for under $100.00.
Lesser-known games, like the Neverhood, tend not to get votes because few people have ever heard of them and because they don't have enough votes on the wishlist to attract attention.

Future, probably. But certainly not near future.
Not only it's fairly recent, but it's still widely sold on retail (unlike most games that are already on GOG). In fact, I have yet to run into a game shop (at least where I leave) that doesn't have a copy in stock.

Meh, Gothic 3 is a bit on the garbagey side. The music was too overbearing and the engine was buggier than the first two games put together. Although I have to say, someone liked Gothic 3 and more power to them, I suppose.

Most, perhaps, but not all.
I've seen Earth 2160 for sale at local stores, to name one. I don't see how that would be a factor at all though. Seems more like a case of getting permission from the publisher, and prepping it to meet the release requirements (stripping DRM, which has already been done, by way of the most recent update, I believe.), etc.