TS Anderson was not that dangerous off his back. He was effective in using his length to keep people in low guard and avoiding damage while running out the clock to get a stand up via the round ending.
That said if you were sloppy, gassed, damaged or had very poor sub defense Anderson could grab a submission. But he was not going to submit any good top control fighter who was not hurt or gassed and who took him down. NOT going to happen.
To the question in your OP, it is a toss up. Anderson certainly would not be in great fear of GSP establishing top position on him which would allow Anderson to be aggressive looking to land a strike on the feet. And if he caught GSP, it could be over. Anderson was a great striker.
But if GSP established early with a TD that he could pass Anderson's guard (GSP was the best guard passer in MMA) and could land some devastating elbows (GNP) then everything changes. Anderson was fully confident against Weidman too even through the first take down. He thought, that like with Sonnen, he could control Weidman and just eat pitter patter punches if necessary until a round ended and they were stood back up. That was UNTIL Weidman landed his first set of GNP and you saw Anderson's eyes roll back in his head. From that point on Anderson had to fight entirely differently both standing and on the ground. He had to fight with desperation to not being taken down again (and risk getting ko'd that way) which impacted greatly his striking on his feet allowing Weidman to outside him easily and KO him standing.
If GSP could land the same type of GNP he landed on BJ, then he wins the fight 50:45. If however Anderson can use his low guard and keep GSP from passing or doing anything in guard I think Anderson takes over the fight in the latter rounds and wins.
At MW I favour Anderson. At WW, GSP is the favorite. At CW its a true coin flip.