Hi Ira and welcome to the forum. I can't think of a product which would be suitable for the gas tank leak repair. However, I would be keen to determine the cause of the leak. I assume the boat is a catamaran. My worries would be that the leak has been caused by chafing in which case it will continue to become worse over time; the other worry is the possibility of ignition, especially if there are wires running through the area where the tank is situated; and the possibility that they could be a factor in the chafing.
I understand the difficulty of carving up the deck and repairing the damage. I wonder if it is at all possible to go in from underneath. I assume the integrity of the deck as a structural element would prohibit the replacement of the decking with an access hatch. In the meantime, it may be a good plan to run the tank dry and plumb in a new flexible tank in a different location. It sounds like a real problem and I hope you are able to overcome it without either great difficulty or expense.
"if at first you don't succeed....Redefine success"!