Some of your Faces are flipped, try finding those faces in Blender and do Shift-N or delete those faces and fill it in. Another method is flipping those faces by doing this:
You may want to triangulate all faces to make sure no face is getting wrong calculations, faces with more than 4 vertices sometimes tend to go a bit weird I think.
A quick solution for gaps is using backculling, I then select every face, and then re-calculate the outside. I think this can be found under “mesh”, then look for “normals” and select “re-calculate outside” if I’m correct.
Not super experienced with Blender but since I’ve ran into this issue as well I also had to find solutions and this one fixed all of my problems.
Awesome fix! I don’t often run into these problems mostly because i am always on the look for wrong Faces/Doubles. But when i do i know its a easy fix.