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

If anda tipe "thirdperson" in the console (in HL1) anda 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 oleh the kerah ring? Also, those glasses have a built-in H.U.D..
BTW, the kerah 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..."