An invaluable resource, and not just because it has a GURPS conversion is
http://en.wikipedia.org/wiki/The_Primal_Order
http://www.rpg.net/news+reviews/reviews/rev_2459.html
'Tis somewhat old but covers just about everything you can think of. From the little things like why even the weakest god is still a god and the strongest mortal is not (something called Primal energy - gods have it and it underlies *everything*) to the big things like why you don't want to fight a god on their home plane to how much energy gods get from worship (and how much they spend to support same) to what happens in those rare cases where a deity is driven to true suicide (trust me, it's bad).
Hard to track down these days but the rules for adaptation to GURPS work incredibly smoothly. The mechanics are close enough it could almost be a supplement.