I have just told you what is wrong.
It's as plain as the nose on your face to anyone who models, i can clearly see multiple NGons on your model.
Unity is NOT GameGuru, Unity can triangulate NGons automatically on import, GameGuru can't, to import an object that has NGons into GameGuru properly you have to convert the NGons into either quads or triangles in a modelling program first otherwise the engine can warp those faces.
This is not a bug, GG is just different Unity etc. It is not GG that is the issue, it is your model, it does not conform to the requirements, in fact it does not conform to most modelling guidelines, as anyone in the modelling world will tell you, avoid creating NGons, they're bad.
Now that we can see your UV it's clear it's not UV, it's actual geometry, What you need to do is edit your model so that no face has more than 4 corners, look at the side panels, you have made them both one face each, they should be at least two faces because the current single faces has 5 sides and should have no more than four sides.
I'm not saying this will fix all of the issues, but this is at least a contributing issue for sure, you also have NGon issues around where the holes in the side panels etc. are (shown in the picture below).
Those circles are made up of sides, each of those sides is adding up to another side on the side panel face, looking at that each panel is a single face with at least 25 or more vertices, gameguru doesn't like more than 4 vertices per face.
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