Fiberglas blistering is somewhat of a mystery as some do and some don't. It's hard to go back 30 years and determine if it was the formulation of the reside, too much catalyst, etc.
I first ran into this on a small sailboat in 1981. The correct fix cost was more than the value of the boat.
Fiberglas hulls leak, just very slowly.
The above pics emphasize the need for a good hull stippling now and then (every ten years?) to check on the hull.
In my area a soda blasting costs about $80/ft.
The layers that have failed have to be ground off. That pic at the top shows more than gel coat failure.
The removed layers have to be replaced. Tricky doing on a hull bottom. Sometimes they lay the boat over on its side.
Then you still have a mess, as the new layers are not really that smooth, so the hull has to be faired out.
Once you have a smooth hull again, it's time for a 4 or 5 coats of epoxy sealer before bottom paint.
Around $12,000 for a 40 footer if you have minor holes or blistering. I suspect the OP pic would be substantially more.