Inclusion of free of cost supplies in gross amount clarified as applying prospectively, with transitional exclusions for pre existing contracts. The Explanation inserted in Rule 3(1) makes inclusion of free of cost goods and services in the gross amount a legal requirement only prospectively from its insertion; however, the Explanation's proviso excludes works contracts whose execution began or for which any payment (other than by credit or debit) was made before the Explanation took effect, in which cases the gross amount does not include the value of free of cost supplies.