Sir,
"Public Sector Insurance Company" perhaps will mean like LIC, National Insurance etc. I think, in respect of such PSUs, service rules applicable may be either as applicable to Central Govt., or specifically framed and adopted by such PSU.
I will suggest you to consider utilising the remedies as are available under service rules of organisation you served.
ALC of State Govt., perhaps will have no jurisdiction in the matter.
You can also consider filing case in Central Administrative Tribunal (CAT) for delay in payment of Gratuity, if service rules of organisation you served so permit.