The highlights and shading is extremely random. Bright spots appearing at the top left corner of parts of the weapon are not how any light source would look like. I would suggest you look at real life pictures, and try to understand how light looks shining on something and how the shadows are formed based on the light. The gun itself doesn't have any texture to it, and when reduced simply looks like a gradient was applied, also due to how the highlights and shadows are placed. See how zombie's glock has a texture feel to it:
Which also brings up a couple other points. Your gun doesn't have nice defined lines, as a gun is made out of metal. For that I suggest you zoom in a helluva lot, and draw the lines. To fix the rugged look of the lines, take a 2 pixel brush and make a thin outline with it. Now, here comes the second point: There's already a glock icon in the section. Now, there is absolutely no unique factor from yours compared to the one already in the database. They both depict the same exact weapon they both are gray. Yours is not as good as quality as the one currently in the database, as well.
You mentioned that there's 500 sword icons, and the reason they are approved are because they are not the same as the others. I would suggest you try a different angled perspective, or give this glock a silver slide. Or even show the gun firing a bullet. The glow simply distracts away from the gun as it is the same or a similar color to the gun itself. I would suggest you get rid of it, or simply find a color that looks good.
The proportions of the gun are also off, for that, I suggest you use a reference and try to mimic it more closely. Glock magazines also go straight across from the bottom of the grip, and don't stick out as profusely as your drew it. The slide is far too long, the trigger grip not wide enough. The slide of the gun is metallic, and the grip and the rest of the gun are made out of a plastic polymer, so those parts should be of a darker gray. The thumb grip doesn't stick out as much either. The front sight of the gun is too far forward aswell. I would suggest overall that you use this as reference:
~Snap