Gun Play is one of Gunslinger's traits.
Killing an enemy with gun play trait instantly reloads the other weapon. If she kills the enemy with primary, secondary will be reloaded, and vise versa. This is can be tricky to use, and actually requires timing and a good situation. And since AI can't use melee weapons and doesn't use secondary weapons unless completely out of ammo or has a flag, Gun Play has almost no benefit or effect for them. However, if used properly, it is one of the strongest traits in the game. The bullets reloaded into the magzine do not come from your spare ammo, indirectly giving you infinite ammo. This trait is used by Calamity.
In the lastest versions, this trait is partially glitched. If you kill an enemy with your primary weapon, it does not reload your secondary weapon. However it does reload your primary if you kill an enemy with your secondary.