In Steam you can't make an informed purchasing decision to only get a game if it doesn't have DRM, because they don't inform you of it upfront.

Further, steam-api is itself a light form of DRM, even if it can be mostly worked around by those with sufficient technical proeficiency by using something like the Goldberg Emulator.

Meanwhile in GOG no games have DRM and you can definitelly make an informed purchase decision that considers the presence or absence of DRM there.

Your argument would have weight if Steam gave you the choice of not buying games with DRM if you didn't want to by informing you of which have it and which don't, but that's not at all the case - you buy the game and only after that do you discover if your brand new single player game phones home every time it starts and refuses to run if there's no Internet connection.