[tbone]

What they said, above.

I'll add the reassurance that core stuff is truly identical for DFRPG and GURPS. That is, you won't find DFRPG introducing a new method for resolving combat attacks, or rebuilding the magic system, or reworking the costs of key character traits. You can dip into GURPS willy-nilly to spice up your DFRP with skill techniques, new powers, or combat options. Or – should you want – rapid-fire electromag gauss rifles. It'll all fit together just fine.

The "difference" of DFRPG is mainly just leaving out a lot of GURPS stuff, for reasons of genre or simplicity. Without making changes that would prevent you from bringing that GURPS stuff right back in.

Actual differences truly are small stuff: details of some spells that need tweaking for the genre, suggested handling of some niche tasks for genre reasons, etc. Stuff that's simple to reconcile.