Well, being a gimp user myself i feel kind of forced to chime in...so...
The image is composed off a random flare, a cs render and a smudged background.
So, the flare shouldn't be there, the render's placement isn't well done and the background is truly messy: some parts are too clustered and sharp (for example near the pistol) .
There's little to no blending too and the color scheme is awful.
My suggestion to improve is to either learn from tutorials (even tho i'm against them) and try any layer type, filter, tool with anything. By trial and error you'd find by yourself what looks good and why.