A really nice editor (with plotting capabilities) is Ncplot. it has the capability to add, remove, or renumber N-line numbers. As well as a number of other handy g-code manipulating features. But the nicest part of it is that it can plot out subprograms, macro programs, and parametric code in Fanuc style g-code. Those features almost make it almost indispensable for complex g-code editing of those kinds of programs.
That said, Linuxcnc really doesn't use N-numbers for anything and it doesn't even care if the N-numbers are out of order, so if they get in your way, just leave them out. I also use regular text editors such as Pluma, and Gedit, also simple IDEs such as Geany or Notepad++ can also be very nice G-code editors.
