Games

Steamworks doesn't support crossplatform or crosslauncher multiplayer, period. Every game that has cross-launcher play between steam and eg. gamepass, gog, epic, etc. has it because they either didn't use steamworks for multiplayer, or they replaced steamworks multiplayer for either EOS, GoG Galaxy, or their own custom multiplayer backend. This is not up for debate, this is something anyone even somewhat familiar with steamworks or gog would already know.

Homeworld Remastered only recently got cross-launcher multiplayer when they replaced steamworks with their shift backend. Same for Homeworld Deserts of Kharak.

Deep Rock Galactic doesn't support cross-launcher play or crossplay between the gamepass (xbox or pc) and steam versions because the steam version uses steamworks multiplayer. DRG support crossplay between the MS store and xbox console versions.

Brutal Legend straight up doesn't have multiplayer on GoG because it relied on steamworks and they didn't bother implementing anything for gog, so they just removed it.

Dying light 1 only got cross launcher play in 2022 when they replaced steamworks with EOS. Previously multiplayer didn't work between GoG and Steam versions.

Off the top of my head, No Man's Sky, Stellaris, Grim Dawn, and Darktide all had to write their own multiplayer backends to replace steamworks to get cross launcher play.

Literally every multiplayer game that was steam exclusive and then releases on epic removes steamworks and replaces it with EOS because steamworks doesn't support cross-launcher play (eg. Rising Storm 2, Mordhau)

edit: GOG docs that specifically point out that steamworks doesn't support crossplay.

In the future you should do literally any research to avoid making an ass of yourself.