GURPS has been my go-to RPG system for about 5 years now (mostly due to the fact that all of my friends who play tabletop games refuse to learn anything else
), so I've tried my hand at converting both Pathfinder and Fantasy Craft adventures/settings into the system.
What I've learned? Pathfinder is a pain in the ass to convert due to the sheer amount of power differential between enemies and characters, and the fact that GURPS characters grow much more slowly even in the most unrealistic game possible. The standard for attempting to simulated TOG and PF is something called Dungeon Fantasy, which is a bit tongue-in-cheek and strips the system down to a more simple format for playing those sorts of games, but even that is more realistic and slower-growing than any PF game.
The settings and rules for the Adventure Companion in FC, however, is also extremely powerful in comparison to PF, but strangely it's easier to convert into GURPS. Priding itself as being able to do anything, GURPS can take, say, the weapon chains from the main book and get them down to a few points worth of Techniques, Perks, and Skills. It's still tough, but it's not as hard as usual.