Refer sec 22 of CGST ACT 2017
22. (1) Every supplier shall be liable to be registered under this Act in the State or Union territory, other than special category States, from where he makes a taxable supply of goods or services or both, if his aggregateturnover in a financial year exceeds twenty lakh rupees:
Provided that where such person makes taxable supplies of goods or services or both from any of the special category States, he shall be liable to be registered if his aggregate turnover in a financial year exceeds ten lakh rupees.
Aggregate turnover defined under sec 2(6) of CGST Act 2017
(6) “aggregate turnover” means the aggregate value of all taxablesupplies (excluding the value of inward supplies on which tax is payable by a person on reverse charge basis), exemptsupplies, exports of goods or services or both and inter-State supplies of persons having the same Permanent Account Number, to be computed on all India basis but excludes central tax, State tax, Union territory tax, integrated tax and cess
Exempt supply is defined under sec 2(47) of CGST Act 2017
(47) “exempt supply” means supply of any goods or services or both which attracts nilrate of tax or which may be wholly exempt from tax under section 11, or under section 6 of the Integrated Goods and Services Tax Act, and includes non-taxable supply;
Interest income is Nil rated supply as per notification 12/2017. Rent income of commercial property is taxable income as per notification 11/2017 but rent income of residential property used for residential purpose is Nil rated supply as per notification 12/2017.
So interest income and rent income from residential property is Nil rated supply and GST will not be applicable.
Interest income is Nil rated supply but rent income from commercial property is taxable supply and if aggregate value of supply is 20 lakh and more than GST will be applicable and all the provisions of GST will be applicable.