I would not use a razor blade unless you are working on real glass (not poly ester) a nice plastic scraper works as well on fiberglass gel coat, and there is no risk of damage to the gel coat. I have always used the heat gun on low (high can damage a gel coat--maybe better a hair dryer, with lower heat.)
There are also adhesive and trim removers such as from 3M which work well.
There will be a shadow, where the gel coat has not aged--usually this can be buffed out with Finesse it II, or with time it will fade to the same as surrounding areas.