To use a ship/field gunner you simply have to select "gun ship or field" in hagar, choose your gunner position with a minimized clipboard and then click the H button on clipboard.
To use GV (ground vehicles, ie panzer etc) go to hagar, right click on the list and select 'vehivles'. Then you get the list of available GVs. You can either spawn in hangar (H button) or at a spawn point away from the field (they are indicated by yellow arrows on the map).
As for gunning in someones bomber you have to join his plane. Do that by typing '.join ' in the radio buffer of go to roster, right click on his name and select 'join'. One advice: ask first before you join someone.