Here is how the sales page works for consigned artworks.
When a work that is sold and its on consignment to the Art Centre from the Artists (normal non purchased work) then there is commission paid when the work sells.
Artworks sold like this don't incur GST on the sale.
C.Rate is the commission rate charged by the Art centre to the artist .5 = 50%
C.GST $ is the GST of that commission because an Art Centre is GST registered and needs to charge it.
C.$ is the commission amount charged by the Art centre to the artist, which is exclusive of the C.GST
So as it appears is correct and the customer/gallery should not see this the C.GST as its really a separate transaction to the actual sale.
When the Art Centre purchases works and then resells they need, they are the obligated to charge full GST and the C fields will be empty and the GST will have the correct amount.