Can't give direct input from experience as a driver since I don't recall ever seeing a cyclist signalling with reflective gloves. But I've sewed bright yellow reflective material to the backs of my gloves and they seem effective enough when I signal a left turn and want following traffic to slow enough for me to reach the left turn lane. In that situation the car lights will be pointing in the general direction of my hand and my experience as a driver has been that there's enough stray light even as high as a cyclist's helmet to make reflectors very effective when approaching from behind.