Breathe out longer than you breathe in
Tension and shallow breathing feed the gag reflex. A simple fix is a slow breath in through the nose for a count of four and a longer breath out for a count of six. Do it for one minute before you start and in every pause. Let your shoulders drop and your jaw hang loose. You will notice that the reflex fires later when your breathing is slow and steady.
Continue with Gag Reflex Fundamentals or read how to deepthroat without gagging.