All fire buttons and keys have been moved into the global set. There are 3 different fire functions there. Fire primary, fire secondary, and fire all guns. WRT to being a gunner, this is how those functions work:
Fire primary- fires only the gun you are manning.
Fire secondary- fire the secondary weapon of the position you are manning (e.g. coax machine gun in tank turret)
Fire all guns - Fires all positions that are slaved to your gunner position. This includes other gunner positions on your plane and other planes in your formation.
It sounds like you just need to map a button to fire all guns and use that when you're a gunner. If you only want that functionality when you are a gunner, use one of the other modes to map that functionality and then switch to that mode when you gun.