The #1 impression of the gog galaxy client compared to the steam and epic games clients is that you do not need it.
Some part of me still thinks that even making GOG Galaxy was a mistake, because by the very way of its existence it incentivizes people to make completely nonsensical comparisons like the above - no matter how clearly GOG advertises that Galaxy is a 100% optional piece of software and not a yet another launcher used to implement DRM.
I want less crap on my PC. Being able to just install and run the game, without any additional launchers (which for decades used to be the normal way to do things), is the #2 reason I prefer GOG to any other game store.