The leak comes from a discovery a data-miner did from Fortnite’s latest patch-- V7.40. You can clearly read in the image down below it says “Gun Game”, as well as the caption “Earn eliminations to make your way through all of the weapons to the end!”.
This leak does not mean that the game mode is officially coming, we all know Fortnite has leaks almost every time they release a new version. Another thing that is not revealed is when the game mode will be coming, with some people already speculating that it might arrive with tomorrow’s update, but nothing for sure.
Here is how Call of Duty Wiki describes the game:
"Each player starts with the same basic pistol. Every time you kill an enemy with your gun, you're upgraded to the next weapon. Anyone killed with a melee attack is demoted to the previous weapon, so watch your flank. Suicides also set you back to the previous tier, most often occurring when using explosive weapons. The match ends when someone cycles through all twenty weapons. Success in Gun Game requires aggressive tactics and familiarity with each weapon in the rotation. Don't worry about dying - just go for a lot of successive kills”.
Obviously there might be players who find this game mode too different from what they are used to in Fortnite’s regular game modes. Epic Games might have an ace under their sleeve and will probably add changes so the game mode flows smoothly with what they have previously implemented.
Fortnite Battle Royale is the FREE 100-player PvP mode in Fortnite. One giant map. A battle bus. Fortnite building skills and destructible environments combined with intense PvP combat. The last one standing wins.
Fortnite is available on PC, PlayStation 4, Xbox One, Nintendo Switch, Android, iOS & Mac.