Hi, cjfranke 

For Purchases/Expenses under the Debits column enter the amount and under the GST column select the relevant GST rate on purchases this will automatically calculate the Credits column with the GST amount ( inclusive of GST/ Gross amount ).

For Sales  under the Credits column enter the amount and under the GST column select the relevant GST rate on sales this will automatically calculate the Debits column with the GST amount ( inclusive of GST/ Gross amount ).

Check the attached screenshot 

Comment back for further clarifications.