But if G0-T0 doesn't leave then how can he destory Tyber's power base?
Tyber can (and does) project power on a galactic stage. The war is all over the galaxy if G0-T0 doesn't leave Nar Shaddaa then Tyber wins by default.
Hanharr won't be bribed? Is loyal to the Exchange? What game where you playing? He (if you are dark) comes to YOU with a plan to find G0-T0 and kill for the Hutt. So yeah, he is not as loyal as Urai.
G0-T0 doesn't need to leave Nar Shaddaa. But his forces (aka HKs, Ubese bounty hunters, Gand, Exchange, Zhug Brothers) will move across the galaxy to hurt Zanns powerbase. Also remember that in order to win G0-T0 just has to kill Zann. G0-T0 is all about endgame.
There's no winning by default. Traya stayed on Malachor and she won. Somehow. Anyways, staying in one place does not make you lose by default. In this case, it'll make G0-T0 win.
What game were -you- playing? You don't bribe Hanharr, he comes with you because he wants bloodshed. He doesn't care about credits, just battle. And siding with G0-T0 will get him many more people to kill than siding with Zann. There's no reason for him to defect.