groze: Sadly, the online GOG community is pretty much non-existent, right now.
To be fair you can't expect things to change overnight... it's going to take probably a few years for GOG to build any type of online community and it doesn't help that Galaxy is still missing basic online functionality like gaming joining or VOIP, etc.
I expect things will slowly change though given enough time... especially if more games include cross-play... which a few on GOG are doing.
Tropico 5 has a good number players last time I checked online, but that was a long time ago... don't have it installed right now.
There is always games like Terraria to play, that game has player hosted servers so just find a server online and connect. A lot of people are playing it.