As reported by Rockpapershotgun:
GOG explain in today's announcement that starting with a "selection of hand-picked" Epic exclusives in a closed beta, they're going to let folks games buy Epic Games Store games inside GOG Galaxy, as well as games from their own store.
Their example screenshot of a store page for The Outer Worlds inside GOG Galaxy notes that "You can install and launch [the game] via both GOG Galaxy and Epic Games Store client", so it sounds like you can skip using the Epic launcher, which would be a small blessing. The screenshot also notes that yes you do need an Epic account, and buying Epic games through GOG Galaxy will add them to your Epic account.
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"The new store inside GOG Galaxy operates like any other store," the company told us. "It means we're selling games and earning a cut out of these sales."