Hello karlh0985,
Thank you for posting here in the Community. I'm here to help you with any questions you may have with GST in QuickBooks Online.
When adding items on transactions, you can select methods (GST, GST Free, or Out of Scope) on the line items. For your visual reference, you can check the sample screenshot I'm adding down below:
For additional insights into GST, such as for the specific items, I'm adding the articles I recommend on this:
The Community has your back, so feel free to let me know if you have any additional questions with the GST transactions.