How do I add GST and AU$?

Jane, can you explain how these costs are figured? With that information, we can help you with the calculations. Please post a link to the form you have online now so we can see what fields we're dealing with. Thank you.

The 10% GST can be added as a calculation field. Please see this documentation: http://www.gravityhelp.com/documentation/page/Gravity_Forms_Pricing:_Adding_Tax

1. Add a Product (Pricing Field) to your form.
2. Select "Calculation" from the "Field Type" drop down
3. In the Formula box, select the merge tag (or tags) you want to multiple by 10%, and add them to the calculation box, then multiply them all by .10.
4. After you've done all this, save the form, and preview it to see if the calculation works.

Example: http://gravity.chrishajer.com/purchase-a-baseball-bat/
Screenshot of my calculation: http://minus.com/lhVqGRwUt9gjB
Screenshot of my prices before GST: http://minus.com/lV752IAFeNfhA

You will see the total is updated with the price plus GST, then if you select shipping that is added to the total AFTER the GST calculation. You can combine merge tags in your calculation if you like, and just add them all together, surrounded by parentheses, then multiple the whole thing by 0.10 GST.

Regarding AU$:
Gravity Forms supports just one currency per installation. The currency setting can be found on this page for your domain: http://example.com/wp-admin/admin.php?page=gf_settings