The math on the data entry screen is simple columnar math.  What is in the Amount column adds to the subtotal.  If the GST is in the amounts that add to the subtotal, the program cannot show the GST at the bottom again because the Subtotal + Taxes = Total.  So the program doesn't show it at the bottom.

With the cursor on any line item, you can select Ctrl-Y to see the line taxes (Sales/Purchases menu, Tax Summary for the line) and you can see the journal entry before you post on the Report menu.