I think GSP is a GOAT but I would argue some of the bullet points in the OP do not hold up.
Defeat every person he ever fights
What? He lost to Serra...
So even if you're just speaking about literal wins, you're wrong.
Then we also have the questionable win against Hendricks, but that's not even worth going into at this point, since Serra is enough to falsify that statement.
Win a belt in two different classes
I'd argue this is not a requirement.
I think GSP was the GOAT before he beat Bisping.
And I feel like if you've fought good competition at your own weight - you shouldn't have to move up a weightclass just to get respect.
To me, beating Bisping was a good accomplishment - but not a careeer-defining one.
I would've valued GSP's venture into MW much higher if he had fought at least one more top MW guy, like Jacare, Romero, Rockhold, Moose or Weidman.
A single fight just isn't enough to prove much.
I'm not gonna completely shit on George's win over Bisping. I still think he was a pretty tough fight. (Even if you think the names mentioned above, plus maybe Gastelum, were better than Bisping, you gotta agree he was at least around a top 7 MW)
It was a good win.
But not great enough to be highlighted as one of the 4 best things GSP has ever done.