This game reminded me of how the early stages for games are. This game gave me an alpha build vibe. It wasn’t super detailed and there are plenty of things that need to be attended to and fixed but the overall idea and gameplay of it was pretty decent. First, my critques.
As said earlier, the UI was not centered, and I would also make sure you are keeping the aspect ratio for the ui so it fits on all devices. Here is a dev forum post just in case you weren’t aware of this. Keeping the Aspect Ratio of your GUIs. The UI text and color scheme could also be better. The text was a bit small and off-centered.
The next thing was that the zombies looked plain and didn’t really have much to them. You could probably look at how other zombies are made and replicate it a bit, at least the body type for them. You can give them ripped clothing and the zombie face. Another thing I noticed was you could easily cheat your way through by defeating the first zombies then just stand back and shoot the others while they don’t move towards you (I did this with the boss as well). I suggest making their range to target players a bit larger. You could even make the zombie system similar to how the game “Zombie Tower” does theirs. After you get to certain area, zombies spawn in specific spots and you can’t move on until they are defeated. I also suggest making a map such as a city, sewer, building, etc. These are just some things I believe could be improved on. I wish you the best of luck!