Just wondering what popular multiplayer games there are on GOG.

Depends on what you consider "popular".
If you're looking for titles with whatever-shite-is-listed-in-the-Top-10-of-what's-hip-with-the-kids-these-days levels of popularity you are definitely in the wrong place.

Stardew Valley and Grim Dawn.

Now "popular" has relative meaning of course. Both games also have full DRM-free multiplayer available (direct connect/LAN) in addition to server matchmaking.

I'd say the Blizzard titles can be called popular MP games.

Many older strategy games had / have MP but were primarily singleplayer, like HoMM series, the Starwars RTSs and the Homeworld series.

GWENT is MP only so that counts.

Then there's the Quake series and Warsow, which you get for free.

The Divinity:OS series has coop, as does many other recent RPGs.

Giants Citizen Kabuto also has MP, though I don't know if its functional today even with stuff like gameranger.

And browsing around, there's always this filter: https://www.gog.com/games?sort=popularity&page=1&feature=multi