Let's ensure that GST is setup in your account so it will be applied on your invoice. I can guide you how to do it.
Go to the GST menu.
Click the Get Started button.
From the Tell us what taxes you pay and how often page, setup your GST information.
Click the Save and Finish button.
Once completed, you can now create an invoice. Make sure to indicate whether the invoice amount is exclusive or inclusive of Tax. Lastly, assigned a specific GST percentage on each line item in the invoice. Here's a screenshot on how it looks like:
I've got these articles to learn more insights on how to manage GST in QuickBooks: