Disclaimer: I'm basing this line of thought on Pathfinder Society and the scenarios/modules ran in it. I don't care what you do in your homebrew setting or if you have an evil GM or any house rules you may have. I'm going strictly off of PFS rules.
There are several factors that come into play, none of which works in favor of your player.
Gunslingers are already amazingly powerful and overpowered (I know I'm going to get a lot of flak for that).
1. They can do touch AC attacks which will dominate almost anything at high levels that isn't specifically prepared for firearms (and few mythical creatures are). This means they can take all the penalties of deadly aim because they will virtually never miss, gaining a huge bonus to damage.
2. x4 critical and with improved critical that's a 19-20 range. A gunslinger can do 100+ damage with a single crit by level 5.
3. DEX to damage (I'm assuming he's a pistolero or a musket master). This is insane as it permits gunslingers to only need one stat for their ranged attacks. One stat that gives huge boosts to AC, initiative (to shoot flat-footed targets), reflex saves, damage, and to hit.
A musket master can only do a touch attack within 50 feet (if she spends a move-equivalent action to gain an extra 10 feet to the musket's range).
A rifle can shoot a touch attack up to 400 feet with penalties. However these penalties are very little because you're still shooting touch which is easy to hit and he probably will get far shot to lower the penalties.
If your player gets distance on his firearm, double the range. 800 feet for a Rifle and 100 feet for a musket. That's significant; that's sniper territory. I haven't even added the Deadeye Grit which only increases the range he can shoot at.
Imagine him shooting touch attacks at 1600 feet for a -15 penalty (not really that big at higher levels and its still just touch AC). This doesn't even include if he's got + weapon bonuses on his guns or buffs.
Example: Group sees a dragon in the sky far up and heading for the group, that gunslinger can probably kill it before it gets close.
The other main thing is that he will only be able to use metal cartridges for his gun, which will cost him 7.5 GP a shot (to make) unless he gets a potion/oil, wand, or a friendly caster just to cast abundant ammunition.
If you still want to do it (and I'm assuming he's a musket master), remind him that he needs to take Rapid Reload AGAIN as the rapid reload feat under musket master specifically says MUSKETS, not rifles.