You can create a Cylindrical Coordinate System at the center of each gear, and add a Displacement Constraint of X=0 because in a Cylindrical Csys, X is Radial.

I would like to know why did you choose Explicit Dynamics to simulate two gear teeth?  What is your simulation goal?

If it was to see the gear teeth turn and go in and out of mesh, you could do that in Rigid Dynamics with a lot less time spent computing results.

If it was to look at the stress in the gear teeth, you would be better off in Static Structural looking at a model with just a few teeth on each gear as others have done.

Attached is a working ANSYS 19.1 model.