Ditto on the reverse running, go slow in reverse and get through the gel. Once you are through the gel chamfer the gel cut with some sandpaper so your saw cannot catch an edge. A hole saw will get hot going through the transom and a cheap one will start to deform from the heat which will cut an oval or ugly hole. Once the pilot bit comes through the inside start form the inside and cut a relief about an 1/8 of an inch or so. Go back to the outside and finish the cut. This will prevent splinters when coming through from the outside to the inside. Take your time and don't let it get hot.

All that stuff. And coat the inside with polyester resin or epoxy. If you keep the boat you'll thank me!
Seal the fitting with silicon or use 5200 if you never plan to remove it.
Gary