The GST on a property sale is 1/11 of the total sale price; if you use the margin scheme then the GST is still 1/11 of the margin.
Unless the GST % amount (like caravan park rentals @ 5%) is different I don't see why you would need to create another GST Code
But if you need to go to lists>tax code list>new
That's correct it's 1/11 of the margin or GST amount * 11 and then GST free (or input taxed) for the remainder, this is on purchase of property. The sale of a property would have the full amount of GST surely.
The BAS will report correctly in showing the GST gross amount (I use GST inclusive figures) and then the total GST free amount (CAF or CAI)