No Suman Sir. There is no such Rule. GST is levied on the "TRANSACTION VALUE." Refer the Valuation part of the CGST Act 2017. MRP includes GST but the Transaction Value excludes GST.
As per the Provision of Section 15(1) of the CGST Act 2017, GST is levied on the "Transaction Value".
Refer the Provision of Section 15(2) and 15(3) of the CGST Act 2017 for how to determine the TRANSACTION VALUE.