From The Document Foundation Wiki
Gerrit: Adding a new Identity to an existing account
To link another identity to an existing account:
- Log into Gerrit with your existing account (use https://www.google.com/accounts/o8/id )
- Select menu Settings, clicking on your name on the very top right corner
- Then choose Identities option form the left area menu bar
- Click the 'Link Another Identity' button
- Type the URL to access your OpenID provider in the OpenID TextBox (i.e. https://launchpad.net/~YOUR-NICK)
- Authenticate with the other identity (click sign-in) and authorize Gerrit to be connected with that OpenID provider
- Log in using the other identity can only be performed after the linking is successful.
See here for examples on openid providers.