First line of the first response hits it pretty good.
Depending on age & material quality the decal will be easy or the other way. A hair dryer or heat gun on low along with patience & a non scratching putty knife of some sort will get the decal off.
The remaining adhesive is the fun part. Yeah, the expensive packaged for the job products work but I like cheap that works. Stuff that might already be laying around. Lacquer thinners dissolve all kinds of things. Acetone is another excellent cleaner that will not harm gelcoat. One more little known cleaner is Naptha. It will remove residue that another cleaner might leave behind.
Finish off with a good marine cleaner wax & the surface should be like new.