Up front caveat: My account is on the pro-piracy instance. This isn't some corporate boot licking bullshit, pirate it all. Just an attempt to educate.
From a purely legal standpoint, purchasing from GoG is still only purchasing a license for a game, no different from Steam, Epic, any other online game storefront, and no different from buying physical.
All game purchases, for decades, have only been purchases of a license. Seriously. Go check the back of Super Nintendo game manuals.
The benefit of GoG is that you don't have to remove the DRM yourself, and they usually include patches to keep old software working on modern machines.
But Steam and other storefronts can't stop you from stripping the DRM from purchases once they're on your machine anyway.