This works for Gnucash 4.12 on a Mac.
Reports > Multicolumn > Custom Multicolumn Report
Click the Options icon.
Set 12 columns.
Choose Contents tab.
Scroll down to Income Statement and add it 12 times.
Click apply and close.
In the screen will be 12 columns of data. Click the link that says "Edit options" in the bottom left corner of each column.
Click the General tab.
Set the Report title to "Name of Month Year" Income Statement.
Set the Start date to the first month in your tax cycle.
Set the End Date to the last day of the first month in your tax cycle.
Repeat 12 times, incrementing the month each time.
Now you have a month-by-month income statement that can be used to populate a spreadsheet that SARS will accept.
Source: original hint from 13 years ago https://narkive.com/37J5CDu9.5