ReynardFox: When you buy on GOG, you own it, when you buy on Steam you're technically renting it on the whims of the developer and the service. I think even when GOG is more expensive you're getting the far better deal here.
Fenixp: GOG - and any other retailer for that matter - can't legally sell you ownership of the game and can only sell you a license, if you really want to go down into technicalities.
If we are already talking semantics:
You are refering to ownership and Nemo dat quod non habet.
But not license in case of GOG, IMHO. as it is appraised as DRM-free, they give you ownership of the copy you purchase.
German high court has decided though, if you only purchased a digital version of it, including the advertisment saying so (like Steam and similar), you have no ownership.
With GOG advertising DRM-free open for debate AFAIK ;)