Some possible ways to report this transaction are:
You can show the value of old gold received as an inward supply liable to reverse charge in Table 3.1 (d) and the corresponding ITC in Table 4 (A) (3). You can also show the value of new gold supplied as an outward supply in Table 3.1 (a) and pay the GST accordingly.
You can show the value of old gold received as an exempt supply in Table 3.1 (e) and the value of new gold supplied as an outward supply in Table 3.1 (a). You can also reduce the value of closing stock of old gold by the amount of exchange and increase the value of opening stock of new gold by the same amount.
You can show the value of old gold received as a purchase of capital goods in Table 4 (A) (2) and the value of new gold supplied as an outward supply in Table 3.1 (a). You can also claim depreciation on the old gold as per the Income Tax Act.