We are only able to support actual Gmail email accounts in Nimble because of the way we can access Gmail's API. Currently, we do not have a way to work around this limitation through Gmail.
However, if you connect your main Gmail account to Nimble, you will still receive all emails that were directed to your aliases and they will associate with your contacts in Nimble, but you won’t be able to send through Nimble under your alias.
If you have any questions, please write us at care@nimble.com, initiate a chat from this FAQ, or join one of our Nimble Onboarding and Best Practices sessions, held every weekday at 9 AM PT.