Relieving Certificates - company saying i have to pay GST on notice pay amount

I worked for an MNC company for around 4.6 years. I have to serve 3 months notice period. I have requested them to adjust my 45 days PL against 45 days and balance 45 days I will work for the company. They have calculated 45 days PL with basic salary and confirmed me that I have to work for balance period. Then I agree to work for one month and balance amount i will pay notice pay. Same way i completed my period and quit the company and joined another company. I have paid my notice period amount after few days of my resignation. Now, after four months of my resignation also they are not in a position to give my experience and relieving certificates. all of sudden when i called to them they are saying i have to pay GST on notice pay amount. This also I agreed and to deduct the same from my bonus. after this also they are not ready to release my certificates.
Experts please clarify
1. Whether PL's can be adjusted with encashment / days.
2. Can i go legaly that they are harassing me??
3. Even after these many days they have not relieved me, can i fight legally that they have to pay me the Gratuity??
As I am not against the company, but people are harassing and not in a position to at least passing the information to ex-employee.
please advice and suggest what best i can do ?????

First and foremost, PL can't be adjusted in notice period. However, you can take encashment against your balance PL.
You cannot go for legal ground in this matter because, when you had given the notice period and expressed your term to serve the same, the receipt of that letter is itself proof of the same.
No need to give/ receive you any other relieving letter, your mentioned date on the resignation whatever you have mentioned, the said date is your last working day in the company. So please don't worry and carry on with your new assignment.

they have adjusted my PL's with encashment and calculated with basic salary. even though i have fulfilled all the formalities still why they are not releasing my Relieving. Can't we claim for gratuity as i have not been relieved by them as on date.