It could be that you have a 20 amp machine on a 20 amp breaker.
Breakers do get old and after a while they will trip at 19.5 amps or less.
When you trigger the gun the machine has a brief surge that will draw over 20 amps (number used for illustration purposes). A weak breaker will trip.
The GFI is a different type of breaker. In the presence of a direct ground it breaks the circuit to prevent electrocution.
Get the breaker tested or replaced. Also test the amp draw of the machine under load, sometimes a motor will be a wee bit sticky to start. This should clear your problem.
Hope this helps? Cheers.