This is what a developer replied about a user asking info about the standalone installer that come with the game bought on the official site.
"It's the same version GOG sells."
So what's the point in buying it on GOG if the other one is the same plus steam key plus more money to developer?
For me personally, it is so that when I finally download the game (DRM-free installer), I don't have to hunt it down from various developer sites, but one place.
But if they offer both the Steam key and the DRM-free installer for the same price from the developer site, I agree that may the way to go for many. That's what I did to the Android Indie bundles, for the Windows versions I both downloaded the DRM-free installers to my private repository, and enabled the games on Steam, not having all my eggs only in one USB hard disk. Naturally I downloaded and installed also the DRM-free Android versions (.apk files).
But, if I have a GOG version, I usually have no reason to download it also from Steam (even for free), so meh.
ANYWAYS, it seems my PC doesn't probably measure up to the HW requirements of this game, so I'll pass it anyway for now. I wasn't expecting it to have "high" HW requirements, considering it is a Dungeon Master/EoB clone. It is as if a turn-based strategy game, or a text adventure, would suddenly have very high HW requirements.
I think I'll play Arx Fatalis instead for the time being.