The gear in SU looks ok. I used solid inspector and it says everything is shiny, but when I export the .stl to my printer software, the center hole is filled and a gear tooth is missing. Can anyone tell me why and how to fix this?
I have attached the .skp file and a screen shot of the gear in the printer software.
The gear teeth aren’t solid. Only the hub is. The teeth have no faces where they meet the hub. You need to fix the teeth or make the whole thing a single solid.
That gear extension was written back before 3d printing became a thing, so it doesn’t make the gear a solid. There may be better ones around.
But to fix it, since you have solid inspector it is quite simple.
Good that it worked for you, but you should look at the model that @DaveR posted and compare it to yours to understand what it is that needs to be done to make a solid.