Many users are asking the same question, and a simple answer to this is it doesn’t matter what email you have used before. You can use any email you want, either the previous one or a new one.
If you have more than one wallet with FRM and/or FRMx holdings, you should use a unique email address when registering and authenticating your wallet in the Gateway.
You can learn more about the process of connecting emails to your wallet in the: How To Sign Up for Gateway and Leaderboard To Access Presales article.