Been trying to find information on the matchmaking because there are a bunch of old mp games I really want to play(like descent) or if matchmaking is even out yet.

If you mean Galaxy only a handful of newer games, and AvP i think. Matchmaking for old games was merely a wish by the community and hasn't been confirmed as a planned feature for Galaxy as far as I know.

What Pheace said. The alternative is to use Gameranger. It's mostly hassle-free; you just need to spend a few minutes to create an account and add people to your Friends list.

The GOG Galaxy page certainly makes it sound like there's a wide support for multiplayer and matchmaking. There supposedly was a list on Reddit of all the supported games, but the links to that list seem to be broken, so I'd love to see an official list from the GOG staff or something, and if the GOG Client could be better at telling you which games it can handle multiplayer for.