GOG is a preservation effort doubling as a store, the other is just a failed business move to "open" PC market from steam, as they like to think.
GOG has been growing, that for sure it now resembles more a store.
your comparison in a little skewed GOG do not release new games nor is considered as a site to get new releases (although, Sony has released some games from the 8th gen on it)
Epic store needs to go against real market places like steam or god forbid the windows store.
I can't speak more about the Epic Store, never used it, and never will.
Why the comparison?