What's the meaning of "gas" here?

Solution 1:

I believe the author is talking about losing their energy after a short period of time

Solution 2:

The sentence is saying that today's MMA/Jits fighters are much more physically fit and trained than "meatheads" of the past, and thus don't "gas out," or "run out of gas [energy]" as quickly. "Gas" comes from shortening "run out of gas."

It is common in martial arts, mixed martial arts, MMA, etc. to say something like:

"I gassed out too quickly; I must not have trained hard enough."

"If you are gassing during [insert event], then you may need to [insert advice]."

You could use "becoming fatigued," "running out of/running low on energy," and "running out of steam" in the same way.