Either full notice period or gross salary in lieu of same is the right procedure. When you are ready to serve proper notice period if the employer is relieving then they are bound to pay you balance proportionate gross salary. While signing the full and final settlement statement you should have noticed it and asked for clarification. Go to your office by person and meet your HR and make a request and ask for full salary against notice period.
If yes, Then no point of asking them.
If No, Then what can I now at this point.
Basically terms and conditions on notice pay should have been mentioned in your Appointment letter /contract of employment letter as applicable in your case. Also if your Company is having certified standing order you can check in it also. While the general practice is Gross salary some company follow Basic salary only.
So it depends on the Appointment terms. Please check the separation clause and then discuss with your employer.