It would make for an awesome 8-9 rounds before Gatti was TKO'd. Remember, Gatti was a very big 135 pounder. With Marquez being the counterpuncher that he is it would allow for Gatti to get some nice shots off, and I do believe that it would take JMM a few rounds to get a handle on Gatti's power. Slowly JMM's great counterpunching would wear on the courageous Gatti and he would finally wilt and be stopped in the 8th or 9th.
Gatti would make a good fight of it and would as ever leave the ring having put in a great effort.Marquez would just have too much for him and would win,most likley on a stoppage around the 9th or 10th.
i also feel that gatti was weaken by the sudden shifts in weight..he is more of a 140 but forced to move up and down for bigger fights
JMM would have Gatti out of there earlier than he did Diaz via one-sided beatdown. Gatti's wide punching and open defense would be a field day for JMM, even with Gatti's power at 135. I could easily see JMM landing at a near 60% rate against Gatti.