just a little thing i thought of because i have no idea what gluon gun m2 does in black mesa (sorry for the awkward view, shouldnt be a problem as far as you have a functioning neck)
So on the 25th anniversary page you can grab the crowbar and hit stuff with it. And if you hit Gordon it makes a metal sound (because hev), But then the same sound is made when you hit his head.
Gordon's head is undeniably made out of metal.