Ministry of commerce and industry of India doesn't provide WPI index for Natural Gas. Is there a way to calculate the same?
Since 2014, this is the official formula to compute the price of NG in India:
The details of this formula, including where the data is taken from can be found here.
To see historical data with this formula (since 2014), see the left panel here. Simply click in the period you want to see. Notice these prices are set for a six-month period.
There is more information about the context of this policy and the different pricing systems regarding natural gas for India here.