One of the scientists in Anomalous Materials personnel facility section mentions this. This is heavy speculation on Devs’ part and I think this piece of dialogue shouldn’t have been included. If he doesn’t wear a helmet, then how does he see the HUD and survive in heavily contaminated areas? The definite proof that he does wear it is in Episode One where he enters the Citadel Core’s main chamber alone because Alyx doesn’t have a suit to survive exposed to it. I know his HL1 model and cover arts depict him without a helmet, but this is very likely just to make him look cooler.
You’re… kidding, right?
That line was written literally to lampshade the entire “helmet” argument and allow people from both sides of it to have it their way.
It’s optional.
You, the player choose whether or not to have it via your perceptions. It’s not confirmation one way or another. It’s up to you.
Forget fundamentalist religion and hardcore politics… the “Gordon had a helmet” people are by far the most extreme.
I honestly don’t care either way. I interpret it as no helmet just because the HEV model doesn’t have one. Maybe it’s a retractable helmet or something that only goes up when radiation is detected? Maybe the HUD is displayed in his special glasses? It’s 100% up to the player.
A few things to think about.
Firstly - that line is intentionally ambiguous, as users have pointed out above. On the team we are split pretty evenly between thinking Gordon does/doesn’t wear a helmet. There are convincing arguments for either side, so we have no strong viewpoint on it to put forward.
Secondly - the entire game is basically our interpretation of the Half-Life universe. There’s plenty of other changes and speculations we’ve made elsewhere that someone playing the game will experience. That’s just how it goes. I don’t think us making a judgement here, if we had, would be audacious, given the entire scope of the project.
Thirdly - you’re saying there’s definite proof that he wears a helmet, thus weighing in on the argument yourself with a definitive viewpoint. You’ve done the same thing you’re complaining to us about, only on the opposite end of the spectrum.
Gotta love the helmet debate!
By Shrödinger’s helmet!
Think valve somewhere stated gordon doe NOT wear the helmet.
I think it’s pretty clear he doesn’t care, doesn’t think about it, and only even comes up with anything for it because people keep asking about such a trivial topic.
Can we all just agree the hud is a holographic projection? Rule of cool? Gordon sees tripmine lasers thru more holographic shit? (HECU never see it coming ;- 
I don’t even know anymore. Personally, I like to theorize of those above, and assume the suit has a built in augmented reality display to keep track of these things. Black Mesa has some pretty high tech stuff, so I wouldn’t even rule out a forcefield/energy shield of some kind. I mean, the suit basically has that feature with the rest of the body.
Gordon puts his helmet on and off.
wait whhaat? =O 
Gordon is really a cyborg. It explains everything, really.
If the helmet is a transparent accessory ?, an energy barrier? …
I know, I’m talking about things that seem not obvious.
Like a shield…
Think about that, the energy of the shield also covers the head and not a helmet is required