Financial transactions / transactions in money (i.e. merely by way of loan given and its repayment) per se are not taxable under GST. However, consideration therefor can be made taxable.
Even if without consideration, supply is presumed between related parties when made in the course or furtherance of business Reference: Serial No. 2 of Schedule - I.
Moreover, one should be mindful of exemption available for 'Services by way of extending deposits, loans or advances in so far as the consideration is represented by way of interest or discount (other than interest involved in credit card services)' Reference: Serial no. 27 of Notification No. 12/2017- Central Tax (Rate) as amended till date.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.