What strikes me about your screenshots and your videos is that your levels, while highly-detailed for Game Guru, show a lot of repetitious entity placement and a slight lack of consistent art direction.
Throughout your pictures, I see the same green and blue cars, the same 751 bus to Waddington, the same red van, all reused multiple times. I get that it's difficult to populate your world with a variety of assets, but some simple texture changes and colour variations would go a long way to making your levels look more organic. You can probably get away with using the same bush or tree 30 times, but something distinctive like a lurid green car sticks out, especially when 11 of those same cars can be seen at once (as in your screenshot of the car park at the abandoned mall).
In addition, the rest of the world has a wonderful decayed feeling to it, with weeds sprouting through the road and foliage clinging to every available surface - very I Am Legend
, very cool! - but lots of your entities look fresh-out-of-the-factory-new. I hate to pick on them again but it's those horrid green, blue and red cars. They look so clean, especially when sat next to other cars that are all rusted and smashed up. If there's any way you can dirty up those textures, add a bit of rust, some dirt, or even just desaturate them a bit, it would help create a more uniform visual aesthetic.
You've clearly got a knack for creating detailed maps, and your use of music/sound design is good too, but by making a few small changes it could elevate this project up to be one of the best Game Guru projects to date