you can do this using the calculations tab of the eCart object
on the calcaultions tab, create 2 new calculations as:
Name: GST
Type : Currency
[Price] * 0.1
Name: GSTInclusive
Type : Currency
[Price] + ([Price] * 0.1)
when adding the display manager to the page, you can select to include these 2 calculation, so that each line item will show the price, the GST amount, and the GST + Price amount.