Here’s a sample of gcode for 2 small holes (in the [negative x, negative y] quadrant):
1B00_ZFB_CP_Base2Hole_T476_AA101.nc (3.1 KB)
Basically you can compare the format and headers for this code to the Mach3 code you have, that should help you determine what needs to be edited or changed with an editor (“notepad++” if on windows or “bbedit” if on a mac). If you need/want a larger or different example, feel free to let me know…
As far as loading g-code post-processed for a specific CNC machine into an application for the purpose of being converted/post-processed for a different CNC, I don’t know of anything that will do that automatically, but that is because I haven’t looked for one. I make my own models in Fusion 360 and use their post-processor (with carbide 3d setting, generic has worked too) for outputting my CAM g-code, so I haven’t had to convert any g-code yet, but it is entirely possible this capability exists in some application…