Gratuity bypass - company acquisition - were made to resign and rejoin new company - what can be done to get gratuity?

Joining Date: 11/12/2014
Resignation acceptance Date: 02/June/ 2019
Old Company Name: XXX (Pune) (Health Care SAP IT client based)
New Company Name: YYY (NAVI MUMABI) (client based)
Hi, I’m working at client location in Mumbai at Reliance Foundation hospital, I worked in XXX Company for 4 Years and 5 Months and 18 days. Suddenly we got news in May 2019 that company has been sold to YYY Pvt Ltd. And we have to resign as without resignation new company won’t take us.
And we were forced to resign in May 2019 but company asked us to write April month for resignation.
New company HR (who left organization after this) convinced us that he will talk to old company for gratuity and will get it done.
Now we are working at the same client location with new company YYY Pvt Ltd but old company is denying gratuity and new company also denied it now.
Old company has wind up its business in India but it’s in existence in California, they planned such way as to not give us gratuity
Are we eligible now & what can be done to get gratuity?

Section 25-FF of the IDA, 1947 lays down that in case of transfer of the ownership of an industrial undertaking by agreement or operations of any law, the employees shall be paid retrenchment compensation.
If there is no specific clause about this in the transfer deed, the new employer, as successor-in-interest, would be liable to pay the same or take the previous service rendered for the purpose of continuity of service. In all probability, the employees would have no knowledge about such a specific clause nor have any access to the transfer deed.
The alleged forceful and back-dated resignation amounts to "constructive dismissal" only.
Better, raise a collective dispute u/s 2(k) of the IDA, 1947 against both the companies.