He has a percentage of the RESIDUALS, the profit percentage for the first two films (more for the second) but he is not under contract for GB because he could be forced to make a crap movie if he were so contracted. He is only connected to the property by the desire of the writers, producers and studio to bring him back for the role of Venkman. In several interviews, he has said that he won't be a part of a third movie UNLESS it was a tremendous script in his view.
It took a miracle, a midnight dinner in the back of an Italian restaurant with all the principals, a promise of a LOT of back end cash and the friendship of the team to get Murray to sign.