[Varyon]

I think the typical method for melee sentry removal (ranged is probably more common, although I could be mistaken) is to sneak up behind and put them in a blood choke. Human beings, even those with Combat Reflexes, simply don't react as quickly as they do in GURPS. I think they also typically don't patrol with their fingers on the trigger guard of a weapon with its safety off.

Assuming the target expected an attack and/or has Combat Reflexes, they're going to start out under conditions of Partial Surprise, being Stunned and unable to act. Assuming you manage to successfully start the choke hold, a Fright Check may be appropriate - unexpectedly losing the ability to breathe is terrifying! If the target passes that and manages to shake off the stun, opting to fire his weapon to inform his buddies rather than grabbing the arm in an attempt to pull it off so he can freaking breathe should probably call for a Will check. Following that, he'll probably need to use a Ready to turn his weapon's safety off, and the GM may call for a Guns check to manage this while grappled (and the attacker should be able to Parry the attempt by shaking his victim at the wrong moment). After that, probably another Guns check is needed to get a grip on the trigger (again, attacker may be able to Parry) before he can actually fire. During all this time, the attacker is strengthening his grip (if using Technical Grappling) and/or possibly attempt to disarm him by shaking him around or whatever (he might also be attempting Strangle, Head Lock, Neck Snap, or similar). The end result is that even if the defender has ice water running through his veins, he's probably unconscious before he can sound any sort of alarm. Against a sentry who does have ice water running through his veins, it might be best to start the attack with All Out Attack (Double), grabbing his weapon/hand with one arm and his neck (in a Choke Hold) with the other. Get his weapon away from him, then switch both hands to the Choke Hold and wait till he passes out.