Mail.app: How to use multiple From: addresses for the same Gmail account?
In Mail 9 (el capitan) this is done by adding the new address as an "Alias" in the Preferences.

Mail > Preference > Accounts tab > Account information tab. Above the "Email address" Field you will see "Alias". Click, and click "edit Alias". Click the plus (+) icon on the bottom left, and add the email address and name.

Of course the address must be properly set up and authorised as an extra account in the Gmail interface itself first.

Gmail > Settings > Accounts and Import > Send mail as. Treat as alias must be on (checked).