If you're using Gmail from the web (ie: Safari or the Gmail Apps) it pulls from your gmail contact list; if you're on mobile gmail, the right hand side of your browser when viewing mail will have a contact/address book icon (delete the contacts).
Alternatively, if you're on desktop version of gmail when viewing mail, you're see it says Mail > click that and change to contacts.
You can also go to the settings to not automatically add a contact (click the gear / then mail settings / Create contacts for auto-complete: change to I'll do this myself).