I don't think it's possible to bend a headstock, but what I sometimes do is hold the top of the headstock with my left hand while pressing onto the upper horn with my right. I dont use a tremolo so this is a nice sounding substitution.
The GW could also mean that you bend the string at the headstock. Actually, I believe this is exactly what they mean. Basically, you do some taps on one string with your left hand while bending it above the nut with your right.