Here's something I've been wondering...I've seen Gordon wear the HEV suit, but never the actual helmet. Does he wear the casco as well? He has telescopic vision and headcrabs can't mount his head, so he must wear it, right?

If tu tipe "thirdperson" in the console (in HL1) tu can see that Gordon doesn't wear a helmet...
But in HL2 the thirdperson model is broken so we can't see Gordon...
But we can presume he wears it

Thought about that too. Perhaps it is a localized force field generated por the collar ring? Also, those glasses have a built-in H.U.D..
BTW, the collar force field also contains the controlled atmosphere needed to survive the toxic gases and breath on Zen, BUT it functions as a two-way sound damper with a defective (Acme)mic, therefore Gordon cannot verbally communicate. "...You're a man of few words..."