Most of the work involved in applying a new layer of gel coat to an older boat will be in the preparation of the hull. If you are hiring someone to do all the preparation work and the application of new gel coat, the cost of the gel coat portion will probably be only a small part of the total project cost. The amount of preparation work necessary will depend on the present hull's condition.
Restoration of fiberglass boat hulls with new gel coat is a boutique business. If you are near an area with a large boating population there is probably a local artisan who provides that service.
A 1963 Boston Whaler boat will have two gel coat colors. This will require more costs than applying a single color.
Application of new gel coat is often not the best approach to restoration. Use of modern two-part paints can produce very good finishes.
A very good presentation on youTube shows the work involved in preparation of a hull for a new top coat. See
https://www.youtube.com/watch?v=pUOlWPvgLfQAs the narrator mentions, the work is "labor intensive." When hiring labor that requires many hours of work, the cost will be proportionately high.