You can play both Epic and GOG games on the Steam Deck, and you can do it with or without the Epic and GOG launchers. Your choice. I did so today, in fact.
The Steam Deck runs Linux. That means anything that runs on Linux, and most things that run on Windows or Android can be run. Including Windows-only software.
Idk why people act in this day and age like the Steam Deck is some crippled device. Unless your desktop runs Linux, your Steam Deck can run more things than it can.