How do I stop gagging when making dental molds?

Nose breathing. You can heave the dentsit spray a topical anesthetic in the mouth then swish around for a minute and spit. That will numb the mouth and can help releive the gag reflux. Also breathe through the nose and stare at one point on a wall.
Breathe thru nose. Try taking rapid shallow breaths in and out of your nose while the mold is in your mouth. Do not breathe through your mouth or try to swallow.