The current implementation has a 90 degree arc being stepped through 1 degree at a time — adding 2D outlines of tools will simplify this down to a single operation when using Rotate Extrude. Similarly, complex outlines such as for roundover tools are greatly simplified by using 2D outlines and Linear Extrude.

Currently, 2D outlines and profiles of tools and shafts are being added:

Once the full complement of such shapes is in place, the updated versions of commands will be implemented.

That said, the program as it stands: https://github.com/WillAdams/gcodepreview should be workable, and it seems unlikely that any of the existing interfaces/commands will be changed (though there is also an effort to add support for 3D printing), so testing and experimentation and constructive criticism by others would be welcome.