There is no gag action when using one rein alone. When you use both reins; the gag cheeckpieces slide through the bit; the bit rises in the horse's mouth, and extra pressure is exerted at the poll by the crownpiece of the bridle.
The pressure at the poll is really the primary effect; along with a secondary nutcracker like effect as the bit rises and the poll pressure increases.
Sorry, don't mean to sound stupid, but are your reins attached correctly? They shouldn't be attached to the bit, but to the ends of the gag cheekpieces. Sorry to ask!