When you start to enter an email address in the To field of a new message in Gmail, the field auto populates with previously used addresses that meet the criteria of the characters typed so far.
You may find that there are addresses that you no longer want to appear in this list and that you wish to delete.
To remove an address, select Contacts in the top left area of the screen
Next select Other Contacts and in the right hand pane place a tick in the checkbox of the corresponding email address. It is most likely that the address is located here unless you have previously contacted it on a regular basis in which case it may be in Most Contacted or My Contacts
Select the More button from the taskbar and select Delete Contact