I'm unclear why this is seen as a significant issue. OK, guns are simple weapons, so anyone can use them, and they act as ranged touch attacks in the first range increment, so armor isn't helping AC.
If they're outside that first increment, regular AC applies. If they're inside that first range increment, they're pretty easy to close in with for melee combat - at which point reloading or firing that gun provokes an attack of opportunity. Or the fighter can back away instead - now he's outside the gun's range increment, but well within his bow's first increment, and both combatants need to hit normal AC's.
Who's using the guns? Other warriors? Well, the fighter can use one too, if he so chooses. Arcane casters? If they weren't using the gun, presumably they'd be using spells, most of which are also range touch attacks (at better range in many cases) or don't need to roll to hit at all.
Maybe the Fighter should consider defensive items that don't provide armor bonuses, but generate deflection or other bonuses instead. Just as useful against many attacks, and more useful against touch attacks.
Maybe give the Fighter the ability to access the Gun Tank ability to modify his armor to be useful against firearms. It seems that could be used in a modified archetype, or alternatively made available as a feat so any class can use it. If firearms are that common, wouldn't this skill be more common?
Some of the problem is that, if firearms are that common and have been around, you would think that the methods for defending against them would also be more prevalent. I think choosing to make a campaign where guns are commonplace should come with some other changes, not just "hey, these guns are everywhere". A product reflecting world design where guns are everywhere seems like a good one but, until then, maybe we have to consider such issues in our own campaign designs. Maybe the price reduction to firearms that comes with more common firearms should be matched by a reduction in the price of Amulets of Bullet Protection. Maybe such protection might also be added to armor or shields, or take other forms. That seems like an easy way to ward off some of those bullets.
Ultimately, the guy with the gun needs to inflict enough damage on that fighter to take him out - before the fighter responds in kind, whether with his own firearm, or with other weaponry.