Gcode Simulator

CNC Machines Shapeoko
6

Most G-Code previewers are free, see the list at:

Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable

There are a couple of commercial ones, and some communication / control programs include previewing functionality, including some of the commercial ones as noted by @UncommonDad

List of those at:

Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable