How does one become a door gunner or gunner on a HMMWV?

Odds are you will be assigned a truck and trailer to work with. Learn that first. Your assigned weapon will be by your unit MTOE. Odds are there is a machine gun in your future. Doesn't mean you will shoot it more than once a year. Learn your real job, get every welding cert you can. Your skill set is far more important that riding around like a German Shepard with your head out the window because it looks cool.

Does your unit have vehicular weapons systems?- If so talk to your chain of command about manning one. If your unit deploys, then there you are- if not then NO. Only systems with the Mech/Armor/Aviation folks have gunner slots/MOS's