GST/VAT is automatically calculated for both Income and Expenses based on your original setup.
Be AWARE that there can be some variances to the calculation of the Tax, so you need to check your Invoicing or your receipts to verify the correctness.
Depending on your region, some transactions, such as interest and bank charges, do not attract GST/VAT.
If the GST/VAT is not correct or should be zero, simply select the box where the GST/VAT appears for that transaction and enter the correct value.