MMA guys with high achievements usually are cross trained. GSP first base was KK and then he learned wrestling, BJJ and boxing, he also had sparred with kickboxers and different styles karatekas etc.
The same Fedor too was cross trained, more than this: had competed in judo, sport sambo and combat sambo before turned into MMA type fighter. Had also paid to boxers, KBers to spar with him.
Most mma fighters train out of mma gyms where they train striking and grappling for mma. Of course quite a few top level guys have bases in other things before going to mma. Some will go out during their mma careers and work with masters of the arts mma takes inspiration from, as GSP did working with roach and others.
My implied point was that GSP is often thought of as an MMA guy in a relatively pure sense in his current fighting style, in many ways being a pretty good ambassador for that textbook modern mma striking with the longer, deeper stance, eclectic kicking but he too has a base in kyokushin and there are more like him that don't always register as kyokushin guys in MMA.