Just by way of review:
Gagging - a completely normal reflex designed to help babies move food they are unable to swallow to the front of their mouth. It isn't a sign of a problem, nor is it choking.
Choking - accompanied by a terrified look on the child's face, and he will start turning blue. Gagging might produce some watery eyes, but the baby doesn't look alarmed, so you shouldn't be either.