The difference between these two altcoin is that they don't have the same algorithm Bitcoin Gold uses Equihash while Bitcoin Cash uses the same algorithm that original Bitcoin uses which is SHA-256, BCH can also be mined using ASIC while BTG can be mined using GPU, BCH have 8MB block size and BTG have the same block size that Bitcoin have (1MB). BCH don't have segwit while BTG have, the difficulty adjustment of BTG is every block while in BCH it is every 2 weeks. BTG aims to make Bitcoin mining decentralized again while BCH aims to solve the scalability issues and replace the original Bitcoin, but it will not really.