Physical stock value is less than book value. First you have to find the reason for difference in stock. It could be due to overvaluation of opening stock if any.
In accounting, stock is valued at lower of cost or market value. So if stock valued less due to this reason then no other adjustment is required.
Further in case of composition scheme, sale is the main scrutiny point. If value of stock is low otherwise than accounting adjustment, there is chance that sales were not accounted. So you must report sales and pay GST.
In this physical stock value is higher then book value. Again first audit your accounting books. Better to check if it is due to opening stock under valued or under reported. The other reason could be you have not accounted any purchase bill.
Under GST you have to check if any sales bill is missed to account and report in GST return.