[whswhs]

I simply don't think it is possible, in a GURPS-like game, to avoid GM rulings during play.

Game mechanics has two basic options. There are closed systems, where there are rules for how you do certain things, and those are the things you can do, and whatever the rules don't provide for can't happen during the game. And there are open systems, where you can do a wide range of different things, pretty much anything you could have a fictional character do in that genre and setting. But if you're playing in an open system, you have more options than any rules system can cover. So when a player wants to do X, and the rules don't cover X, the GM has to make something up.

Of course there are fair and unfair ways to do this. But a completely detailed simulation of a game world—let alone of how things work in every imaginable game world—wouldn't be a book; it would be an encyclopedia, at a minimum, and learning to use it would be about the equivalent of taking a PhD, or worse. And believe me, I understand the aspiration! But I also would like to run games sometime. . . .