You can't simply obtain free G-coins with a hack or bug. If you do happen to find something of these lines it would be classified as illegal misuse of game. A Paypal account only holds the money not the generic object (for this case it would 'G-Coins').
Some online C4C services (I highly recommend you don't try any) allow you to get money on your paypal account through completing online ad's. As mentioned, these services sometimes don't pay, and they are highly unreliable.