Replay Attack: If you execute a transaction on the BTG chain (when this is possible), someone could execute the same transaction on the BTC chain, since all hashes would be the same. He could simple steal your BTC. The same goes the other way around.
A protection on this attack (as implemented for BCH) would be to add a value (just a flag or modifier) in each transaction in order to create different hashes.
BCH already implemented this - but I do not know the status for BTG.