He is gaining both, with how much a man like him or any other athlete eats he is going to gain weight. It is going to be BOTH muscle and fat, period.
That starvation mode that he enters pre-fight to get to his weight class will cause a loss in muscle AND fat.
You want to know how to stay lean and eat that much? It isn't a secret, you work your ass off. You put in the time in the gym with the weights, that means you leave it all in the gym when you are in there. You would be surprised what continuous training and consistency can do for a body.
Additionally, he has been doing his "job" for how long now? With the people that are PAID to train him and his own knowledge I am sure he has an idea of what his body needs. When you get there you won't have to sit there and think about bulking/cutting/WHATEVER. But you need to take the years and put in the time.
Class I Total @ 198: 525/335/600 (1460)
"No citizen has the right to be an amateur in the matter of physical training.... What a disgrace it is for a man to grow old without ever seeing the beauty and strength of which his body is capable."