"...although on the surface DoomRL seems to have levels and ToHit/ToDam, in the inside it's 100% GURPS"
Except that GURPS is a d6-based system, and DRL uses d4s, d8s, etc.
D20 is nowhere near as flexible as people make it out to be, but GURPS ain't the "holy grail", either.
The only Multiverse RPG system that's worse than GURPS is RIFTS.
It's better to just write up a quick system, if you want to run Doom as an RPG. I'd be willing to do it, if anyone was interested.