old gold manufacturing in gst

Dear As per Valuation Rule in GST the The GST will applicable on Transaction Value (The selling price)

Let's suppose Customer gives U a gold for RS. 100/- & your transaction value (Selling price) is 500, then GST will be applicable on 500/- & RS 100 will be reduce from total amount 500+GST

Here suppose customer brings 50gms for remaking to 60gms. we then give a Receipt showing the New ornament to make in 60gms. Details will be shown like this, 60 -50= 10 gm×3200.00 @ gm gold rate +making charge 2500.00=34500.00+1.20%vat 414.ooTotal =34914.0000
Then as perG-11/12 we still follow manufacture thru goldsmith.
Above is the practice which were doing during vat regime.
My question here is does the same practice we have to follow in gst or is there any change. Because we had a doubt in gst regime that tax had to be collected on whole value(60gms)of manufactured ornament.
Hope you now understand my doubt.please clear us to again re start old gold manufacturing of customers.
Thanks! Dinesh Banthia