To show the GST amount of each item sold in the invoice, you need to add a column into the template with the GST amount field. You should use the Detail.Tax and not the Transaction.TaxAmount field.
The Detail.Tax field is the GST amount of each item sold, Whereas the Transaction.TaxAmount is the GST total of the invoice.
The "Detail" table captures the information of each item sold, such as stock code, item description, unit price, quantity, GST Code, GST amount, etc. Whereas the "Transaction" table captures the information of the invoice transaction, such as the Reference number (invoice number), Date, Total GST, Gross, etc.
Therefore, to show the GST amount of each item you should use Detail.Tax instead of using the Transaction.TaxAmount, which is the GST Total of the invoice.