Learned experts/members have given valuable advice. Kindly follow it.
Kindly check your appointment letter. Notice pay @ basic pay, gross pay, CTC must have been described in it. If the company has not mentioned notice pay in lieu of notice period then company can not demand.
However usually companies do not commit such error, therefore in FNF statement company has demanded gross salary then it should be in line with notice pay described in your appointment letter. Company has also stated in FNF that you may tender notice pay @ gross salary so that relieving letter can be released.
It is believed that amounts payable by you to company are more than amounts payable by company to you, therefore you may tender notice pay @ agreed terms and attach a covering letter to demand for relieving letter, and any other certificate/letter/form etc. which are still pending to be supplied to you.