You can buy a Wax & Grease Remover at an auto parts supply store or just plain old Naptha. Lighter fluid will work as well. None of these will harm gelcoat. Wipe it on wet and follow with a dry soft towel to remove. All these evaporate rather quickly.
Acetone is a little on the strong side for me, and if the surface is hot, could stain. The Naptha based wax and grease removers are what is used professionaly around boat and auto refinishing shops. That or the new generation water/citrus based wax removers and degreasers.
It really depends on what's been applied to the surface. The thing with soaps is they are made with surfactants which basicaly are a wax type of lubricant that can leave a film on the surface. There are many high tech Polymer waxes and surface sealants that soap won't touch as far as taking them off the surface. These Polymer's actualy chemicaly bond to the surface and are not easily removed.
I always use rubbing alcohol to prep a small area for a sticker. Just rubbing alcohol and elbow grease for the small stuff. On big decals I use alcohol to prep the are then regular windex, but I super shine the area until its almost sticky itself. Big stickers don't even budge when you do it this way, even the little parts.