Definitely sounds like gelcoat to me too if it is the transom. Shouldn't be primer there either, just gel and fiberglass. Shouldn't be difficult to fix. Order white gelcoat from Formula. The other thing is you could use any old white gel since either will likely need tinting due to the age of the boat. I recently did a couple repairs on the transom of my '07. They were done using gel ordered from Formula. The small repair on the transom just below storage door matched perfectly and you would never know it was there. The second was right where the round part by the transom shower meets the swimgrid. You could notice VERY slightly that it was whiter where it meets the non-skid but I mean VERY slightly and 99% of people would never notice unless you pointed it out. I suppose I would still order the gel from Formula to be safe. After the cost of that (approx. $100 the only cost is labour from the fiberglass guy. If there is no real filling required all he has to do is spray on gel, let cure, then sand and polish. Most fiberglass guys if they are good should do and only charge you three to four hundred tops. Good luck.
