Hi howellsroofing,
 
Currently, making a GST code default to an invoice is unavailable. You can, however, create a recurring transaction template. This is used for tasks that are repeated often.
 
Here are the steps:
 - Click the Gear icon located in the upper right-hand corner.
 
 - Under Lists, click Recurring Transactions.
 
 - Click the New button beside Reminder List.
 
 - Choose Invoice from the Transaction Type drop-down menu.
 
 - Click OK.
 
 - Enter the Template Name, and choose the Type (Scheduled, Reminder, or Unscheduled).
 
 - Choose the Product/Service, enter the amount and select the GST.
 
 - Click Save Template.
 
For more information about GST and recurring templates, you can check these articles: 
If there's anything else you need help with QuickBooks, please let us know. Thanks.