I made a tutorial on how to make gears in SketchUp. Basically, you can just make a circle and then add teeth around it that are all the same size. There is probably a better way to do this, but I just guess the amount of teeth until the spaces in between the teeth are roughly equal to the size of the teeth. I made my teeth 3mm. two of these gears were printed in PLA and they appear to be holding up well.
https://www.youtube.com/watch?v=NmUBMxbccnc&t=2s