FWIW I have a '92 vintage 2520 and the underside of the deck is only 'painted' with polyester resin - not gelcoated. I am led to believe that the Parker decks were glassed on both sides in the years to follow ... but remain unsure of exactly when. In regards to the impact to your deck, or that you're looking at, it is hard to assess the potential longevity.
If the bilge remains dry and no condensation builds up (ALL boats 'sweat' from within!) ... it could out-live your ownership. FYI, do a search on my name and "dry bilge" and you'll see various ways I've come up with to ensure the boats airs out and remains integral.