I have Return to Castle Wolfenstein on GOG and Wolfenstein: Enemy Territory on Steam. I don't like splitting game series unless I have to and because of that I would love to play Wolfenstein Enemy Territory on GOG. The only question I have is if the game supports crossplay between GOG and Steam or not and I can't find the answer online.
Yes, it's not really "crossplay" per se, but it doesn't matter which version of ET you use. The only exception are 2 servers that still use PunkBuster, so you need the old version of ET before Steam and GOG which still has PunkBuster included to play on them. For all the other servers I'd suggest getting
https://steamcommunity.com/sharedfiles/filedetails/?id=2800452649
ET:Legacy has native support for Apple Sillicon