How to prevent gagging when the dentist makes a mold of your teeth?

Difficult: It is difficulty to prevent the gag reflex if it is triggered by something the the back of your mouth/throat. A topical anesthetic can be sprayed but does not guarantee stopping the gag reflex. Fortunately the mold process goes quickly.

Breathe through nose: If you breathe through your nose, you will keep your airway open and not trigger your gag reflex. It is all about mind over matter. Anything you can do to take your mind off what is happening in your mouth, you will not gag. You can also ask your dentist to numb the back of your throat with some topical anesthetic. Spray type topical works best for this.

Lots of ways: As an orthodontist i take impressions all the time. Try going to dentist 1st thing am, don't eat or drink before appt, sit upright and even lean forward a bit when impression taken, breathe thru your nose, topical anesthetic rinse or spray. Nothing's foolproof. Talk to your dentist about your concerns and he will be better able to support you.