Each orders GST status is set in the field GST.
If true, the GST % stored in the TaxRate field will be applied to the order.
The GST amount can be shown as an inclusive amount shown after the total with each line item showing the inc-GST amount, or each line item can be shown as ex-GST and GST line item shown before the total.
Each delivery country has a GST setting which controls whether GST should be applied to transactions shipped to that country.
If a country has GST applied, but the order is not placed from the Default Country for the website, then no GST is applied.