I would say GGG loses to Kessler, pick em fight with Froch. Probably does stop Dawson. Bika is another fight I think GGG wouldn't have an easy time with. He would be facing fighters much bigger and stronger than what he's used to.
I think the best version of Kessler would beat the best version of GGG that we have seen, unless GGG goes on and shows more against better fighters in the future.
No way GGG stops Sakio Bika, Mikkel Kessler or Carl Froch. Those guys were never stopped. I think Kessler would beat GGG.
Froch offered ggg a catch weight, their team said no, but the catch weight was 173 so don't blame ggg really.