sync a custom app with google account

I have a CRM online product. It has an addressbook and calendar. I want to create a service within my product that allows the user to sync their addressbook and calendar with their free gmail account. would allow them to choose What folders to sync sync to gmail only sync from gmail only sync both ways Sync would not erase contacts sync would not overwrite but append contacts info. I would provide cpanel to an empty site and give you address book database structure and calendar database structure. You would create sync to that database to a sample gmail account that you would create. We would test there to see that it works and my programmers would take from that working model to the actual product.