I'm not sure how many centuries ago but a Gou'ald queen grew opposed to how her brethern were acting. They have genetic memories and the queen, Egeria, bred new Gou'ald to fight. They call themselves Tok'ra and unless it's in extreme circumstances, won't take a host by force.
About eight years ago, while trying to evacuate a planet under Gou'ald attack, I was taken over by a Tok'ra symbiote. Unfortunately there was an assassin after her and he nearly killed us but she used the last of her strength to save my life. Interestingly, naquadah is also a protein marker and I still have some of it in my blood. So I can sense other Gou'ald/Tok'ra symbiotes as well as use some of their technology.