Just wondering if it is and whether or not anyone still plays online, trying to find a GOG game with active (or not dead) multiplayer.

Unfortunately,I don't know a definite answer to your question. I think other members who have seen your post may not know for sure either, which is probably why your thread didn't get any replies for a long time.

Having said that, I assume that PK multiplayer is completely dead. When I start the GOG version of PK/BOOH, and click on Multiplayer, it still says "powered by GameSpy," (GameSpy is certainty completely dead), and zero multiplayer games appear in the space designated to list them. At the very least, multiplayer is dead "out of the box."

The reason I am unsure if multiplayer is 100% dead is because it's possible there are a few hardcore PK players who have modded multiplayer to work and still play the modded version in 2017. But if such players exist, then I don't know the procedures for connecting with them.

TL;DR: Let's assume PK is multiplayer is dead, but someone can feel free to prove that claim wrong if it is.

Apparently it's possible to still (? - given the age of the post with the server list, it's entirely possible most, if not all, are now completely offline) play multiplayer in Painkiller via direct IP connections.

http://pkzone.org/gamespy-multiplayer-server-down/