That's not actually true, according to GOG itself, as of some months ago when GOG released a server on which to play Wolfenstein: Enemy Territory, and along with that, they announced that GOG will now be offering online-only multiplayer games going forward.
And here is a GOG store page for that game, which has no singleplayer: https://www.gog.com/en/game/wolfenstein_enemy_territory
GWENT of course is also an online-only game with no singleplayer that has been on GOG for a long time.
Online-only and multiplayer-only are different things. Gwent is online-only game, it requires connection to master sever. Wolf: ET requires no such thing, you can self-host your own servers, play on your friend's server, public servers or local network. Or install bots and play offline. It's perfect example of DRM-free multiplayer-only game.
Both games you mentioned are free. I don't give a shit about free online games, so GOG could easily put 100 games like that and I wouldn't care. GOG can even make a deal with valve and put "Counter-Strike" here for all I care. But SELLING
an online-only (DRM'd) game, that's a different story.
Page for this game is removed, seems it's not coming here anymore.