GSP is the goat regardless of this fight. He may or may not win this one, but moving up to beat up a bigger man after you've been gone 4 years doesn't do anything to hurt your legacy. He will remain goat no matter what.
Yeah it will be interesting to see if continued to get even better during his time off, or if father time (and ring rust) show up. If he wins though, things get interesting. Does he stay at MW? If he does, could he compete with a guy like Rockhold for example? If vacates and goes to WW, then there's another division that suddenly becomes a clusterfuck.
If bisping fights one of the top contenders (even Luke again), he will be, in my eyes, someone who still lost to elvis sinocic who dominated him and yet, bisping was still gifted the decision