Even though the model already looks complete, there are a few things you need to do to prepare it for game use. It's a list that's the same for any asset you create from now on. Here's what you need to do:
Remove the hidden geometry and layers
Remove unseen faces
Explode geometry to remove groups and components
Purge unused materials
Check if all faces are facing outward
Move files and textures into correct folders
Compress or resize textures
Save in a recognized format
It's really quick to do, and well worth the time getting right. If you start distributing assets with any of these left undone, you will pay the price by being labeled an amateur.
First of all, check that you haven't hidden anything and that all layers are visible. As a rule I never use layers in SketchUp, but if you have used layers then you need to check there's nothing on a hidden layer that you have forgotten about.
Go to View ¦ Hidden Geometry.
If anything appears, right click on...