Before syncing with Gmail for the first time, go to your computer and log into Gmail.com and make sure that you don't have any duplicate contacts. Even though it may appear as if you don't have duplicates from your contacts list, Gmail may actually be masking the problem. Read more about it at https://playaapps.zendesk.com/hc/en-us/articles/226469488.
If this is your first time syncing, and you can treat either your Gmail or iPhone contacts as your "master" list, I would suggest using either the "Gmail Contacts -> iPhone" sync mode or the "iPhone Contacts -> Gmail" sync mode. Otherwise, perform a 2-way sync. After your first sync, I would generally recommend using a 2-way sync.