I selected keypad 1 for "next Weapon" and then keypad 1 again for "Change Gunner weapon". Note the problem: "next weapon" function is not available until you hit the reload key. Then it works for one time only.
I also found out that when this happens the "previous weapon" key does not work consistently too.
If you select a different key for "change Gunner weapon", then the "next weapon" and "previous weapon" work fine.