IMMAF by far. They're far bigger and more organized than GAMMA. Also they are affiliated to the UFC and BRAVE.
GAMMA is somewhat unorganized. Aside from MMA bouts, they also have bouts categorized as "striking MMA." Striking MMA is kickboxing with MMA gloves. Those bouts don't count on fighters records. Fighters who competed in GAMMA tournaments get to compete in both MMA and striking MMA bouts. One of GAMMA's biggest flaws is that they allow fighters who competed professionally to move back down and compete in amateur tournaments. That shouldn't happen. Once you compete professionally, you're not eligible to move back down to compete as an amateur.