Careful work with a razor blade should get the edges to lift and start peeling away. It could be a slow tedious process, just be patient. The heat gun will help as well, but as mentioned, be careful with the paint under and around.

One thing I’ve not seen mentioned is a solvent I’ve used a lot over the years: Xylene.

Found at most hardware stores. It targets adhesive when the decals are already removed, it will not penetrate the decal. Thing is, xylene does not affect paint at all, only adhesive. When they remove vinyl wraps from cars and buses, this is what they use to do the final cleaning.

My best tip when using it is to keep turning the rag so you don’t keep dragging melted adhesive back I’ve the area you’re trying to clean.