Symptoms

The following error is displayed when attempting to login to JIRA Cloud:

"Your Google Apps account email is already in use by another Atlassian Cloud account. Please contact your administrators or log in with your Atlassian Cloud username and password."

Cause

The account(s) in question was created in JIRA Cloud before connecting it to Gapps, and now the information does not match between what you have in your JIRA Cloud instance, and the details for Google. 

Resolution

Proceed by following instructions here:
Managing Users and Groups with Google Apps