Going to guess it’s about shading. Based on the look of the model in Blender it has flat shading and you haven’t prepared it for smooth shading, which the game engine is probably using.
Also some of the faces are facing the wrong way (normals) and game engine hides those that have their backs facing the camera
Edit: few words about screenshots
Screenshots help you to describe a problem but those also give out information about what you have and how they are shown. Because they relay information, might as well put as much as you can fit in that space with few simple and quick moves. Screenshots alone might not be enough to answer your questions, only .blend file might tell enough about the model but you might not be able to share it (which would mean extra work to include a example model that you can).
Your screenshots are full scale and not cropped, they show the model, which is very good. But you could include more or use the space more wisely.
This one includes 5 pieces of information: render engine which affects display modes, blender version, scene statistics, the model and what display mode is used. Total of 14.2 % (green). Areas included in the image that don’t have any useful information or don’t even have anything to do with the thing 61.3 % (red). Other areas don’t include information and might not be easy to use for that.
Here in comparison I’m using this screenshot to tell about the problem I have with how the nose gets subdivided.
And the information it gives
- 5 pieces of information you also had
- Subdivision result is the biggest part of the image, that’s the part I’m having trouble with
- Shows the mentioned subdivision modifier
- It’s a modeling question and the problem I have might be because the way I have constructed it, I’m showing a wireframe in solid shading viewmode of the whole head and a closeup of the nose. Had to adjust viewport camera lens to do that
- Outliner shows the objects that are in the viewport. I duplicated the model to show two versions of it and outliner tells there is a material on the other (affects how it’s shown), and can differentiate which is which by looking at the viewport selection and/or naming.
I’m also using as much of the image area as I think is necessary for clear viewing instead of filling it with things that don’t matter at all (empty editors, tools or properties shelves that don’t have anything to do with my question). Took the screenshot by pressing ctrl+F3 which leaves out the window title bar that has personal information and any other bar outside Blender. If I would think my problem might have something to do with the operating system, I would tell what it is in text. As a result I’m using 48.6 % of the image area to relay information and the rest are areas that would be hard to fill with other information I could think of and do so quickly. I would include a .blend anyway in this case.
All of that just by using few extra seconds to organize the view for a screenshot.