Immediate resignation during holiday not accepted, Salary and other amounts due towards employer.

Dear Sir, I was working as an UGC approved assistant professor at a private engineering college. I had wholeheartedly worked there for five years 11 months even though salary was highly irregular there (sometimes 6-7 months). I got offer letter from another reputed institution during Diwali 2019 holiday, then I informed my HoD on phone and resigned immediately on email and joined the new institute immediately as there was an urgent requirement and I did not want to miss it. Pending salary at that time of resignation was of four months (July to Oct 2019). I could not serve any kind of notice period. They refused to accept resignation by mail and asked me to join immediately stating that I am a good teacher so students relying on me, complete the semester and then they will accept my resignation formally. I had already joined the new institute. I have also given previous college a written resignation. The remark given by HoD in front me was only "forwarded to principal". However, later when I met principal to convince , I found bigger remark that my semester is incomplete and work remaining and all on it. This remark is not written by HoD but the principal. However principal is saying its written by your HoD and he can't accept it as its showing my remaining work. In short, they have neither accepted not rejected my resignation. I met them 4 -5 times and tried to convince them for releaving letter and due salary but they didn't entertain me at all. That Principal is threatening me to inform at my new workplace and ask them how they joined me without releaving letter from previous employer and he will also send a letter to me that I am absconding from work. When I met principal last time in the last week and requested for may salary, he asked me to submit fresh resignation letter and told that submit it and then they will think and do whenever they want as I am not their priority. My due towards previous employer are: 1) Three months (August, Sept, Oct 2019 salary)- July 2019 salary was also due but they credited to me in February 2020. 2) Examination remuneration from May 2017(University provides this in advance to college to reimburse to professors, yet its not given) 3) PF 4) Gratuity Note: Its clearly mentioned in my appointment letter that either party can terminate the order by three months notice or three months basic salary. My expectations are cut my three months basic and release my due salary and other amounts as well as releaving letter. My questions are: Shall I give fresh resignation letter as they're demanding? Can he legally inform my current workplace and take action against me? Can I take action against him, if he does so? Can I get the gratuity? They haven't given to a single person who has left so far. I am also afraid that I will get worse remark in releaving letter. (Though, my tenure was very good there and I have never got any memo or adverse remarks for anything) Kindly Help.