I think those people are just less surprised when something happens. I don't think they are unlearning how to gasp, but it takes a bit more to trigger it in someone who's already seen worse than the rest of us want to imagine.

Similar to above commenter, I'm just flinging poo, but

Those who are trained in the tougher situations are, I imagine, more desensitized and therefore don't do the egads! sort of gasp. I imagine they probably don't necessarily need to rely on that burst of air because they'll take a purposeful deep breath before heading into the fray.

That being said, I think professionals do still gasp. It's probably just not something one's brain really catches onto. But it is a startle reflex - so if a surgeon is squirted in the face with blood, if there's a close call with a firefighter, if a cop walks up to a car and has a gun out under their chin.... I imagine each of these will get a little gasp at the very least.

I have some experience in the ballpark of what you're referring to and would say it's somewhat situational: For example, I never used to think twice about joggers but after I started doing Muay Thai every time I hear footsteps closing in fast behind me I get an adrenaline dump.

One time I almost decked a jogger who passed real close, but as I was pivoting to bob, jab, and jump back outside of their range, I actually saw them and my hand went from a fist to a wave. I don't think they even realized what almost happened, but I felt so fucking stupid.

I even quit wearing headphones when going for walks to prevent getting startled like that, but I guess the flip side is that I'm more difficult to jump now (which was kind of the goal of learning MT in the first place).

First responder training/experience can also make you hypervigilant when it comes to certain things, so I think it's more fair to say, it just changes what can provoke the gasp/startle response.