A whole GOMAD might be a little much, but the point here is that the goal should be to build muscle. You are skinny, and while you are worried about a somewhat jiggly tummy it's doubtful that you or anyone else will care about that if you pack on 10 kgs of muscle.
How to do that? Your boxes programming sounds pretty good and will work, or to get there faster you could switch to a pure strength program.
As for food, maybe 1/2 GOMAD? Really just eat enough so that you start seeing your weight go up. I'd shoot for 2-3 kg per month if you are still doing metcons. That is fairly gradual which should allow you to pack on pretty clean weight. Of course I'd say that's a minimum. Faster would probably work out well too.