The Division does not work exactly like Destiny.
It's not guaranteed that you will gets drops rated higher than your current gear score.
Any item that is Gear Score 256+ is already at the highest possible gear level available within the game: World Tier 5, Item Level 34.
The Gear Score is just a basic indication of how well rolled the item currently is, so a GS260 item is not necessarily *complete trash* for a GS283 player, it simply hasn't reached its full potential *yet*.
If the item has the basic properties that you're looking for, but isn't quite max gear score yet, then you can simply improve it at the Optimization Station.