I've fixed many of these over the years (not just G3). Aluminum boats have been made with wood transoms for a long period of time. In earlier times, they didn't even use treated lumber. Later, the treated materials they used contained chemicals that caused issues with the aluminum. In the boat manufacturors defense, they usually have no clue of the chemical materials or compounds they get when they purchase the wood materials. Each company that makes the treated lumber uses slighly different compounds that can oxidize aluminum.

We stopped using Marine Grade plywood on our replacments a couple of years ago. The price, weight, and material compounds just didn't make sense. Besides, reading the fine print talks about the sunlight directly effecting the shrinkage of the materials. Marine Grade tends to have more moisture in it. Couple that with sun, heat and humidity (all things found on a boat) and you can see the problems.

We switched to the newer AC2 treated materials. Lighter, stronger, less harmful chemical makeup.

Like others said, it doesn't hurt to ask them. They might have a fix for you. If not, you could do it two ways. Do the work yourself and change out the plywood with new material or..... purchase the metel and replace it with metel. Another good option is to utilize polymar material. Good luck.