Acetone could be a mistake. There's no need for it, to remove the adhesive
. Plain naphtha or an "adhesive remover" will take it off, without any risk to the gelcoat
or other finishes. I'm not sure that acetone would be equally safe, you might want to check that.
Using a good 600-grit wet sandpaper (the cheaper brands often have irregular grit grades in them) probably will "polish" the transom, but it might be left rougher and more porous than you'd want, leaving you to do the final work
with 800-1200-2000 paper/polish to get the finish back to a nice surface that won't hold dirt.
Best to make Real Damn Sure before getting into that stuff, it is always easier to wait a few days, than to refinish things.