Not a GB owner, but owned wood boats most of my life. Also union shipwright, worked in yards and owned a yard. I live and cruise on an 83' wood boat.
To stop the rot, it has to be dry, fix any leaks from above. Then you cut out the panel where there's rot or remove the whole panel. Sometimes it's easier to do the whole panel. Besides, the structure behind the panel will probably need some replacing. It also gives you a chance to upgrade your wiring, add insulation.
If you don't replace all the rot, and it gets wet, the rot will spread into new wood. There's something different with plywood today, compared to the plywood of many years ago. It rots much easier. Now I seal any plywood after the final cuts are made, but before I install. I seal both sides and all edges.
There are utility tools that make cutouts easier. They make 5 1/2" and smaller circular saws, easier to cut a line on an existing panel. Also utility tools that have various cutters for small spaces or next to an edge. Having the right tool can save many hours. What's your time worth?
