Heat Gun
Get yourself an inexpensive heat gun at Home Depot or the like. I don't think a hair dryer has the heat to help out much, but you could try it first. Use the low heat setting on the heat gun, and don't overheat the gel or the decal, just enough to make it soft at peel off.I've found Goof-Off works a little better than Goo-Gone, and I always keep a can around as its gets a lot of different stuff off very easily.I had the shadow problem on my boat (on the transom) where I removed the name recently. The gel is "plateau'ed" and was a lot shinier after 20 years of the name decals being there. I couldn't get rid of the "plateau", but 3M Fiberglass restorer and wax seemed to do a good job blending the dull areas in with the shiny.If anyone knows how to remove the "plateau", please let me know.Good luck,Tim