I buy everything I can on GoG due to lack of DRM. If something is not on GoG, I buy from Epic simply because they pay a bigger share to developers than Steam. When I buy a game I want that money go to the devs, not middlemen.
GoG also integrates well with Epic, so I can have all my games there.
I support gaben simply because of what he has done for linux gaming. Epic CEO is openly hostile to whole ecosystem and that's why his company wont get a penny from me. And thats the joy of the whole PC gaming industry - we have a choice of who we want to support and how we want to support, and in the end we, as a consumers, will win, because of competition.