You install it from the in-built discover app then log into GoG/Epic from the respective options when you open it up. No faffing or messing about required.
If you want any other launchers THEN things get a little more complicated I believe (I've not tried it)
You can then open heroic (from desktop or Steam OS mode) and open/download any of your games from there, or you can add the games as non Steam games and open directly.