one of the achievements says “kill x enemies with re-programmed turret”.
are they?
If you pick up a turret from behind and carry it along with you, it will fire at anything (soldiers, aliens) that stand in front of you, until it runs out of bullets.
It appears Gordon is a simple-minded chap. He re-programs turrets by picking them up and pointing them at enemies, uses his MIT education in theoretical physics to get jobs pushing carts, and is better at using a crowbar than pretty much anything else.