The short answer is yes. Changing a transaction or customer in Gazelle will trigger the corresponding transaction(s) to be updated in QuickBooks.
Changing a customer's contact information in Gazelle:
If a customer's information has changed in Gazelle the next time you sync with QuickBooks Gazelle will automatically update your QuickBooks customer information to reflect these changes (i.e. Customer Name, phone number, email address, address, etc...).
Changing the default billing contact in Gazelle:
QuickBooks does not have the concept of multiple contacts per customer record. Instead they have the concept of 'Sub-Customers' which functions differently than Gazelle's secondary contacts / billing contact. These features are not in conflict with each other but they are fundamentally different so we follow QuickBooks documentation which allows you to manage Quickbooks sub-customers in QuickBooks; and Gazelle secondary contacts in Gazelle.
Syncing 'Alternate Billing Contact' changes in Gazelle:
If you add an 'Alternate Billing' contact in Gazelle after an invoice has been synced with QuickBooks; the next time Gazelle syncs to QuickBooks, we will assign the original invoice over to the new alternate billing contact.