No, it's far from being light and it's pratically very different from GOG. Once you've downloaded the game of GOG, you can do whatever you want with it, and play whenever and wherever you want. On Steam, you have to be dependable of an internet connection, the availability of their servers, and your account in order to play the game you legally paid for. On GOG, you "own" the game. On Steam you "rent" the game.

As I said, they are working on it, with Heroes V being the first game included in that system.

The bits I bolded are the vital differences. You also have to run Steam every time, I don't see how that can be considered 'light'.

Finally, the similarities you see only work if you consider downloading and installing the same thing. Whenever I buy a DRM-free game, I download it straight away even if I don't install it straight away.

Then I can install and play when convenient to me, even if I'm without an internet connection.