What is the red G warning light on a 1971 beetle?

It is for the Generator (or Alternator). If it comes on while you're driving, it usually means the fan belt broke. If you do not pull over to the side of the road right now and put the new belt you carry - you should ALWAYS carry a spare fan belt!!! - on the car, your engine will melt because the cooling fan is bolted to the end of the alternator shaft.