Supposedly, there is a debate running on whether or not Gordon had the helmet at all. He is never portrayed as wearing a helmet in the artwork, and the suit in it's docking station lack one, but on the other hand, Gordon would need it in order to survive some of the hazards he would encounter.
And as far as I know, there was never intended to be any helmet art in HL2.
But that means you could take a bit of creative license and make your own!
Bad news, fella's. I got 3 trojans overnight, so I'm having to completely restore my computer. I'm not sure if the file still works, but once I get everything fixed I'll check it out. See you all in about a week.
Technically, the whole 'Gordon's Helmet' argument is kind of annoying. People say that he doesnt have one because the art never shows him with one, and the player model doesnt have one. But then how does he breath in hazardous environments. How does he take bullets to the head.
And Blue Shift is not officially canon, so no-one really knows for sure.