Steam also allows DRM free games, it's probably just not very popular with game developers.
One advantage of GoG is that they seem to actually care that old games on their store are actually working on new Windows versions, while on Steam it's more or less a gamble (at least you can return broken games though).
One advantage of GoG is that they seem to actually care that old games on their store are actually working on new Windows versions, while on Steam it's more or less a gamble (at least you can return broken games though).