I just checked the "Bionicle Encyclopedia Updated" and the Ga-Matoran entry say, "All Ga-Matoran were female." Interestingly, all the entries for the other five elements make no mention of gender, although the Av-Matoran entry says, "The Av-Matoran tribe includes both males and females." Since the updated encyclopedia was basically just a copy paste of the original we can assume that the earliest source for gender locking was from 2005.