Control who’s automatically saved to your Gmail Contacts
If you email someone but you haven't added them as a contact, Google Contacts automatically saves their email address in your "Other contacts" group. Their email address will show up the next time you email them.
Stop automatically saving contacts you email
On your computer, go to Google Contacts.
At the left, click Other contacts.
To select contacts, point to their names, then check the boxes that appear.
At the top, click More Delete Delete.
Source for this article: Google Help Center
Wise Agent Blog - Google Sync