50,000 * 5%x where x = Gavin's level
This way if Gavin is level 1, the zombies would be 50,000 * 5% and if Gavin was level 2 the zombies would be 50,000 * (5% * 2) which can also be seen as 50,000 * 10%
Each time Gavin levels up the zombies increase by 5% by using the equation 50,000 * 5%x
Hope it helps you :)