I’m not too familiar with the mechanicals of the Gunsmith, but I’d say it would probably be easier and cheaper to repair the gearbox.

I have an Elec-Trak 36V battery-powered ride-on mower (often referred to as that daft electric tractor). That has a single-speed electric motor yet still requires a gearbox to provide the required drive speeds. There always has to be some way to get the drive from the electric motor to the drive axle.

The electrics are really simple with a few solenoids that control the on/off state. For example, the mower deck or other implements which have their own drive motors cannot be activated without the main traction drive motor running first.

Another issue is the weight of the batteries, although these are getting smaller and lighter. In some situations, the battery weight will aid traction yet when extra weight such as implements are attached and added to the weight of the batteries this can be a hindrance.

Also, don’t forget the cost of a drive motor and the breathtaking cost of batteries!