The net income on a cash P&L for a quarter does not balance to the G1 figures less GST on the GST Summary Report - why?
This could occur if you have some sales transactions that were categorised to GST Free or Out of Scope of GST.
If there were some transactions that were coded to GST Free, they would appear in the GST-Free sales line on the GST Summary report. If they were coded to Out of Scope, they wouldn't show up in the GST Summary report at all.
I recommend running the P&L report and selecting the income amount, which will bring you to the P&L Detail Report. In another tab, open up the GST Summary and select the Net amount for G1 amount, which will bring you to the GST Detail Report. Compare the two reports side-by-side and identify which transactions show up in P&L but are missing in the GST Summary.