Hello darhillcpa,
Welcome to the Community. I'll be happy to assist you so you can accomplish your QuickBooks goals in a timely manner.
To get started, I'd like to make sure I'm on the same page as you. That said, could you elaborate on what you mean by override the GST on a receipt? Are you looking to edit the tax amount?
I look forward to hearing back from you in the comments below. I'll be on standby.