The heat gun and plastic scraper will work great... but this is one case where a hair dryer may be the better choice, and even then careful with the heat.
Just enough to soften the vinyl and the glue being careful not to start to soften the gelcoat
Scrubbing the cold residue with ordinary hand cleaner, softscrub, or a heavy solution of carwash soap and scotchbrite.
Follow this with a good coat of wax and gelgloss.
The results will likely be very nice until the sun shines JUUSST right on it and you can still see faint evidence of the decal, because the decal had protected the gelcoat under it for the last 24 years.