There is a good chance that charging the batteries externally
won't do a thing but then you've not lost anything just giving it
a try.

I don't know if lead-acid gel cells are susceptible to sulfation
as are liquid-filled cells but that is a usual cause of failure from
liquid batteries being stored without a charge being maintained.

I don't want to discourage the OP from trying to recharge the battery -- be
sure to cover them with something thick and heavy, in case they rupture or
explode (I'm not trying to be funny) -- but when I recently pulled out a
Coleman fluorescent lantern powered by an E640 gel battery, I found it dead.
The several-years' lack of charging had allowed the battery to die.
Lead-acid batteries are fragile beasts.