Good day, jenny25.
The GST out of scope will show on the drop-down depending on the selected transaction.
The option to add a GST is unavailable for Transfer transactions. This is only available if you select either Expense or Cheque.
Please refer to the screenshots for your reference:


You can also check out these articles to learn more about GST FAQs:
If I can be of any additional help, please tag me directly.