Gst to be applied on which amount?

Give invoice to the value of 5gm. Bcoz, That's your sale....

yes u r correct mr Raja

But sale amount is on new 2gm gold value. we are not selling 5gm gold.

u are selling 5 gms. and purchasing 3 grams.

u should raise invoice of 5 gm.

Yes.
You can dealt received gold as purchase and Your supplied gold as Sales. That is the simple way.

Mr. Raj Waghadkar...
How you say "sale amount is on new 2gm gold value"...?
I can give example in your way.
1gm is Rs. 10000.
You receive the gold 3g from "CUSTOMER". But, You ll Can't give/calculate it's 30000. You less some other in that Old Gold. After ll be calculate 20000.
Then you give 5gm...
You prepare the "CASH MEMO" as follows.
5gm gold = 50000
less old = 20000
Balance = 30000.
In this way you can set off your billing at your way....
Am I right....?

OK Raja ji but gst should be calculated on which amount?