[Kromm]

You may find Targeting Armor (GURPS Low-Tech Companion 2, p. 25) of value. It provides rules for cutting the straps of rigid armor, causing it to fall off.

Flexible armor, being worn like clothing, has to be pulled off like clothing. Pin the target and then take a long action (serial Ready maneuvers) for seconds equal to half the armor piece's usual Don stat; see Donning Armor (GURPS Low-Tech, p. 102). Do not divide time by 4 for an "assistant," though – this is unwilling! Still, a pin buys you 10 seconds between attempts to break free, which is enough to pull off something with Don 20.