To remove an unwanted autocomplete email address in GMail, remove the unwanted contact record.
Select "contacts" from the drop-down menu at the top left.
search for the unwanted contact in your address book by typing their name (or email, type it as you usually do in the to: field) in the search box
open the contact, then use the "more option" menu 
at the top middle to select delete.
Once the contact is deleted, the autocomplete in the to: field for that contact name/email will no longer occur.