“Certificaion” implies a statement from an Authority. BTG and BTC are decentralized cryptocurrencies, so there are no Authorities - but there is a blockchain.
You can, however, use a blockchain explorer to confirm that you sent BTC to Coinbase before the October fork date, and you can check your Coinbase account history to verify that you held a BTC balance on your account at Coinbase at the time of the fork.
If you had a BTC balance, that means Coinbase held BTC in your name in their wallets, which means their wallet addresses were credited with BTG at the time of the fork.
Your ability to withdraw that BTG or be credited with that BTG is entirely up to Coinbase; they were the custodian of your BTC when you sent them there.