[Pseudonym]

They are nearly binary compatible. DFRPG is mostly a streamlining of GURPS mechanics, so if you find a reason to add in mechanic x from GURPS to DFRPG because it was taken out, you won't have much if any heartburn. I think the only things that might be directly opposed to integration with DFRPG, in hindsight after thinking about this for a while, might be DF8, a big section of its enchantment costs and equipment tables conflict with the enchantment costs in magical items, and the equipment values that have been borrowed from the revised (and generally better in a lot of people's opinions) values in Low-Tech as opposed to Basic Set; and DF13, again because the loadouts are somewhat tied to equipment from the Basic Set.

Other changes abound, but assuming you mean you have all the foundational GURPS books, and you desperately want some removed mechanic like the ability to make Druids Shapeshift into something besides some reconnaissance animal, or extra effort in combat (I love turning my FP into a cheap +2 to active defenses!,) you won't have any trouble at all.