If the input to this is rotation on the sun gear, what is the output?

The output is the carrier rotation, right?  The planet and ring gears are just there to move the carrier.

Why are you solving in Transient Structural?  What is the output you want from the analysis?

If you want to know the stress in the gear teeth, then you don't want rotation, so you would solve in Static Structural. Apply a moment to the sun gear and have the joint load on the carrier revolute joint to ground be a rotation of 0 degrees. In the solution, you will probe the joint to obtain the reaction moment on the carrier revolute joint and you will have the stress in all the teeth generated by the frictional contact.

If you don't care about stress in the teeth and just want to create an animation of the planet gear rotating, then use a Rigid Dynamics analysis. Input a joint velocity on the sun gear revolute joint and watch the system rotate. This will take a lot less time to compute than solving Transient Structural.