The question shouldn't be "Why should this be on GOG", the question is "Why shouldn't it?"
In my opinion, Oblivion was better than Skyrim. Or at least I believe that, as I played through the whole game (Main-Quest and Side-Quest), at least twice, as far as I remember.
Skyrim? I finished only the Dawn DLC, maybe the story once, but that could be false memory thanks to YT_Videos.
Anyhow. The magic-System was way better, and even the leveling system, was in my opinion better. Hell, you could level fighting by watching others fight!
You already have the GOTY-Edition of Oblivion, which I have it.
The remaster should be right next to it.
Oblivion was great. Be it traversing into a picture, talking with Lucien (R.I.P.), the story of the Grey Prince, even the Count of Skingrad was a great chap to chat with.
While many of us know, that Morrowind was probably the biggest stepping stone in the early games, I believe that it was Oblivion that actually helped Bethesda to achieve greatness, thanks to Oblivion's modability and the Mod-Community.
We'll see if the Remaster can follow suit, and if it does, it definitely should be on GOG.