I disagree that Lesnar will gas. Guys usually gas for one of two reasons: (1) Lack of conditioning; or (2) Anxiety.
Lesnar's conditioning won't be an issue. Wrestlers are usually the best conditioned guys in the Octagon.
His experience on a big stage with the WWE will also help him with the anxiety, only in the sense that he won't be phased by being in the spotlight; however, what he can't prepare for anxiety-wise is the racing of the heart that accompanies someone trying to take your head off. No amount of friendly sparring can substitute for "bad intentions" fighting.
If Lesnar fights smart, he'll use his boxing lessons to learn to defend himself and close the distance and hopefully, he won't think he's Muhammed Ali after a year of hitting the mitts. If he takes the fight down, he's gonna be a nightmare and won't gas. But if he stands too much and takes a few shots, "Rich Franklin Panic Mode" may set in and then no amount of conditioning will help him.