You should read official announcements from the blockchain.info blog.
Eventually, blockchain.info will enable the withdrawal or exchange of bitcoin gold coins. Just as Bitcoin cash has been enabled recently.
But, if you want access to Bitcoin gold right away (once the Bitcoin gold wallet is available) only thing you will need to do is import your private keys into a new wallet. You can do this at any time after the fork, nothing will be lost. And yes, you can export private keys from blockchain.info wallet (whether it is an old or new wallet is irrelevant, new type of wallets will give you a recovery phrase and by using that you can get your private keys)