Answer for GST on rental income
If you are letting it out for residential purpose then it is totally exempted from GST irrespective of your Turnover goes above Rs. 20 Lakhs or not and registration due to that is not required. But if you are letting it out for commercial purposes then you are required to get it registered if the turnover from such income falls above Rs.20 Lakhs.
In your case assuming it that you are letting it out for residential purposes. Hence, you are not liable to get registered under GST.