Gcode is an ascii text format that is viewable and editable in most plain text editors like MS Notepad. It can be viewed in CAM programs like Mach3, but Aspire generates the gcode files using specific post processors for certain machines.
Depending on the software the gcode is generated by it may or may not have line numbers. That depends on the post processor that is used to write the code for a specific machine. Many people hand code their gcode files for fairly simple projects but a million plus lines of code is not unusual for Aspire projects involving 3D models.
In Mach3, Machine codes (M codes) - M3 is spindle on clockwise, M4 is spindle on counterclockwise, M5 is stop spindle rotation. There are online tutorials on writing gcode and there are many books on the subject.