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.
The journal entry will show on the GST Report (eg GST Detail Report) as long as a) one line has a GST code associated with it or b) the lines have different GST codes assigned. If there is no GST code on the journal at all, it will not display on the report. Similarly, if the entry has the same GST codes on both lines it will not show as these codes zero or balance each other out.