From looking at the Ownership Statement that you emailed through to me ealier today, it's most likely that the real estate agency has selected the "Compact" statement style for this particular Ownership Profile. 

The GST Summary section which you requested to display on the Ownership Statement for the "Compact" statement style is only available if the Ownership Profile has been set up as "Registered for GST: Yes". 

So based on the information you provided and advised that this landlord owns a Residential property, and that landlords can't charge GST for Residential tenants, it's most likely that the Ownership Profile for this landlord has been set up in Property Tree as "Registered for GST: No", hence the GST Summary section isn't displaying on their Ownership Statement.