The shocking development in the potential billionaire brawl went down on Monday ... when Elon replied to GSP's public pitch to lend a hand in beating Zuck to a pulp in the Octagon.
"I'm a huge fan of yours and it would be an absolute honor to help you and be your training partner for the challenge against Zuckerberg 👊😁," GSP said on Twitter ... to which Elon responded, "Ok, let’s do it 🔥🔥."
It's a massive deal for Elon ... considering GSP -- the former welterweight and middleweight champion -- is arguably the greatest mixed martial artist of all time. In other words, Musk will be learning from the best of the best.
GSP -- whose background is karate -- trains at Tristar Gym in Montreal under head trainer Firas Zahabi ... one of the greatest minds in MMA.
GSP's record is 26-2 ... with his only losses coming against Matt Serra and Matt Hughes. He racked up wins over Nick Diaz, Michael Bisping, Dan Hardy, BJ Penn and Johny Hendricks.
Zuck also has some star power in his corner -- Jon Jones recently offered his services to the Facebook co-founder, but it's unclear if he's going to take him up on it.
Elon teaming up with GSP provides more evidence the fight may actually happen -- and as TMZ Sports previously reported, Dana White told us both sides are for real about getting in the Octagon together.
A fight date has not been set just yet ... but this thing is really picking up steam.