I have only visualized STL files in Rep host, so I’m not much help there. I have been using this site to visualize my cuts and burns: https://nraynaud.github.io/webgcode/. You have to open your gcode file, copy all, and then paste it into the window and click simulate. Not real detailed, but good enough to see what it’s going to cut/burn.
ESTLCAM also has a simulator for looking at .nc files.
Hopefully some other folks will chime in with other suggestions. I would like to find some other options too.