Someone that was BJJ World Champ in Gi that was also an mma world champion of a major organization?
Rickson by armbar!
Hmm... I believe Murilo Bustamante was a BJJ Gi World Champion and he held the UFC world Title at Middleweight.
Rico Rodriguez was a BJJ World Champ in the Gi (though not sure if it happened at black belt level) and a UFC Word Heavyweight Champion (he was also an ADCC Champion).
BJ Penn, BJJ World Champion in a Gi. Has held UFC World Titles in two different weight classes, welterweight and lightweight, for the UFC.
Just for starters...
Would some of you consider Royce?