.gma file comes with the chart and it's the proof you bought it for your plotter: it's the file that unlocks the chart for our plotter.
If you bought it you dont need to unlock it . If G2 options dont appear ,may be the BC isnt compatible with your GPSMAPS 6008-
Have you took a look at devices compatibility list ?
If you upload the file we can check better what can be the problem ,even because we dont have that BC : it should be nice if you share it