When the GST functions are enabled (Options, Misc page) an additional GST column is shown on the Payments page.
The amount of GST to be entered is the amount which is included in the payment. That is, the payment Amount should be the total received, including the GST amount. The image here shows a total payment of $110 including $10 GST.


Also note that ...
•  When adding payments to an account where GST is charged, when each amount is entered, one eleventh of this will be automatically inserted as the amount of GST included in this payment. The figure can then be manually adjusted if required. 
•  When adding payments, the b (balance) shortcut will insert the total amount of GST still outstanding, whether or not it is one eleventh of the amount of the payment. 
•  The amounts of GST charged, received and owing will appear on the top of the Payments page for reference. 
