Hi Ianinfinance
If you want to do a general journal tick the box purchase, do a credit to the expense account that original entry went to coded GST, and the do a debit to the same expense account coded GST Free.
The memo would be 'to correct entry processed as GST instead of GST Free.
The other option if you haven't already reconciled the payment in your bank rec (which I assume was done as an ABA file), is to first delete the total ABA file, go in to prepare electronic payment, drill down on the entry, change it, go back to prepare electronic payments, make sure to date it correctly to when it came out of bank, then choose Bank file, once you get to the window when you usually save it just cancel out, as you have already sent it off to bank.