I couldn't find a G code viewer to work on my Mac so I wrote one.
I designed it for looking at files intended for milling PCB, hence it only distinguishes between tracks above zero and below zero. As my mill is only small http://www.thebox.myzen.co.uk/Hardware/CNC_Conversion.html
it doesn't do vast areas either.
On the positive side it will handle G0, G1 G2 and G3 codes, a lot of viewers don't handle G2 & G3 commands. It will also handle the I&J commands associated with the circle commands as absolute or relative modes depending on the G90.1 or G91.1 commands. However all the main coordinates are in absolute mode only and it only handles mm dimensions. But hey it is a viewer and it has a zoom so maybe you don't have to convert the main size units anyway.
It is available to download here:- http://www.thebox.myzen.co.uk/Mac_Apps/Processing.html