This should be the last thing you should think of doing with your passphrase or secret keys.  It is not ideal and not recommended to store such sensitive information to your Gmail account.  I have a friend who used to store his details online his Gmail account but the unfortunate happened, his account got hacked and the hackers accessed his secret keys to his wallets all and moved his funds till today those funds has not been recovered. So therefore, storing your wallet details online should not be a safe haven because anybody who has access to your Gmail account can undo you by stealing your funds. If you have noticed and from what I have read through here, you have been advised to write down your secret keys on a sheet of paper and store it where only you can access it secretly when ever you want it as it is the way to having access to your funds.