The Deed as it reads:
Gunslinger’s Dodge (Ex): At 1st level, the gunslinger gains an uncanny knack for getting out of the way of ranged attacks. When a ranged attack is made against the gunslinger, she can spend 1 grit point to move 5 feet as an immediate action; doing so grants the gunslinger a +2 bonus to AC against the triggering attack. This movement is not a 5-foot step, and provokes attacks of opportunity. Alternatively, the gunslinger can drop prone to gain a +4 bonus to AC against the triggering attack. The gunslinger can only perform this deed while wearing medium or light armor, and while carrying no more than a light load.
Now, what if that 5' step gives you cover against the ranged attack? Do you add the two AC bonuses or if total cover is gained do you negate the attack completely?
