"CAN GPA BE USED TO SELL A LAND???"
Of course, it can be.
As for your case (Nalagarh) in particular, you need to read your POA document carefully to see if such a power/authority was given by you to your friend.
The Supreme Court case you are talking about is 'Suraj Lamp & Industries'. The Supreme Court did not (and it never can) hold sales by POA as invalid.
It was a slightly complex transaction consisting of execution of a POA, a 'will' and an agreement to sell, all in favour of the same person, in an effort to give the transaction a colour of 'sale', that was held invalid.
In any case this judgment does not help your case.