Link iPhones contact list to GMail app instead of native mail app?
Nope, you can not change the default Mail Client. However, you can make Google the default directory for your contacts. With iOS7 just login with your Gmail account in Settings -> Mail, Calendar, Contacts. After this scroll down and search for the column "Contacts" there you should the default account to your gmail account. If you can’t find the last one, you probably have only one account with contact sync capabilities.