The Fuel Locker on your boat is part of the deck mold. Access to it is easily gained in the space behind the table area. Take some sidewalk chalk and mark around the area. Then using a hose try to heavily wash the compartment from the outside. where the chalk is gone you will find the leak.
The Catalina 22 New design is unique in the fuel compartment in that it is completely sealed from the inside cabin. This was an improvement to keep fuel vapors from the cabin space. Generally surface cracks in the gelcoat will not leak into the hull. Gelcoat is simply a thin egg shell type of coating over the glass laminate layers. In the area of the hull you are looking there is no core, it is simply laminated glass that is sprayed with gelcoat as a top finish. Leakes in this compartment are rare and yours is the first I have ever heard of if it is really a failure of the compartment. Generally the rub rail leaks a bit at the hull to deck joint but that puts water in the bilge and not in the inner liner where the fuel compartment would leak to.
If you can confirm that the leak is in the floor of the compartment you will need to also check for any rotted glass laminate layers in the leak area. If the glass is weak, spongy, or looks rotted then you will want to cut out the bad area and lay up some new glass. You can finish it with gelcoat or paint as a top coat if you wish. Flex seal is not a repair. No future owner wants to deal with a rubber coating that will be peeling in a few years. The fuel compartment generally gets rather dirty and so to even get a good adhesion you would want to rough up the gelcoat a bit. In my opinion your better off fixing this correctly the first time than to deal with a shotty repair that will undoubtedly get worse over time. The compartment is not structural for the hull but if you are leaking water you could also be leaking fuel vapor (heavier than air) into the cabin space which makes for a miserable nights sleep.
If you don't already have one the Catalina 22 tech manual form the C22 National Sailing Association ( not Catalina Direct) is a great resource for all sorts of repairs and troubleshooting tips. You may consider purchasing it for your future maintenance needs. Hopefully this helps and if you have pictures of the area perhaps myself or others can help direct you to proper repair materials or process as well. Let us know how it goes.