When recording transactions in MYOB Essentials, the type of transaction determines where it will appear on the GST Report. For example, a Receive Money/Sale(Take Payment) transaction will result in a value in GST collected (Sales and Income), whereas a Spend Money/Bill (Pay Bill) transaction will result in a value in the GST Paid (Purchase and Expenses) section. Having a negative line on that transaction will reduce the value of that Tax value i.e. a negative on a Sale will reduce the Sales value.
For more information please see my post on Receive money including expenses paid