Interest income is include in aggregate Turnover

A lot depends upon the nature of interest i.e. how is it earned and where it is to be disclosed?

In case the interest is from a customer for late payment of invoice, then GST becomes applicable and it has to be included in turnover and to be disclosed under 4A or 4B of GSTR 9.

In case it is earned from FD's held then the same is nil rated (Sr. No. 27 of Notification No. 12/2017 - Central Tax (Rate) dated 28th June, 2017) and has to be disclosed under 5E.

Hence based upon the nature of interest earned you may disclose in the correct part of GSTR 9.