Yes, there will be a large current going from G's battery do D's battery. You'll usually get a spark when you connect the last jumper cable.
There are two potential problems:
- The spark ignites hydrogen escaping from the battery. Rare, but apparently possible.
- The current overheats the jumper cables. Don't buy cheap jumper cables that are too thin.
To mitigate the risk: be decisive when you attach the last jumper cable. Don't slowly approach the terminal and allow sparks to fly for too long, get it on quickly.
By the way, it's recommended to have G's engine running before you connect the jumper cables. If you leave the engine off until after connecting the cables, G's battery will have to start G's engine while there's a large current going to D, potentially not leaving enough current to start G.
The large current is also the reason that a jump start is seen as a last resort: the current inrush is not healthy for the battery. If at all possible, use a battery charger instead: this charges much slower, but puts less strain on the battery.