Yes, if your Google account is already added to your iPhone Settings with contacts enabled, and you're attempting to sync the same Google account with this app, you must disable the "contacts" switch to prevent duplicates and other problems. Otherwise, each Google contact will appear twice in your contact list.
To disable it, go to your iPhone settings > Accounts & Passwords, find your Google account, and turn off "Contacts." It will warn you that your Google contacts will be removed from your iPhone, and that's safe to do as they will remain at Gmail.com and will be synced using this app.
Video showing how to disable a Google contacts account:
Please note that syncing a different Google account with this app than the one(s) configured in your iPhone Settings will not create a conflict.