Welcome to the Bunnings Workshop community @rnedward. It's brilliant to have you join us, and many thanks for your question about gas BBQs in enclosed spaces.
You'll need to refer to your BBQ manufacturer's guidelines for requirements. If this is a fully enclosed alfresco with no ventilation, then you couldn't operate a gas BBQ in that space. Typically, the area can be enclosed on two sides, but the other two must be fully open for ventilation. If it's enclosed on three sides, at least 30% of those walls must be open.
It's important that you also have the required minimum distances to combustible material. This includes 1000mm to the alfresco ceiling and 430mm around the BBQs perimeter.
I've included the instructions from one of our BBQs below so you can have a general idea of what might be a similar requirement. As mentioned, you'll need to check your unit's guidelines for requirements specific to it.