Hey,
I Played your game and then here are my Ideas and "Critisisms".
So, the map i pretty empty for the moment and finding zombies was pretty long.
Zombies walk on each others and there are some little glitches.
So my Ideas :
Maybe adding some animations to the gun for reloading.
Maybe reducing the map size to give player a little more time with zombies instead of being alone.
I think you didn't use Navmesh but you should take a look at it for your zombies.
Here you go.
Good luck
Bye