I've used toilet bowl cleaner (acid wash) for years. It doesn't harm the gel or decals (so far). Takes off everything but scratches and oxidation with very little effort. Warning, if you have a concrete drive, keep it good and wet, or it'll etch your concrete.
'06 X2 MCX
"I understand why some people may not want to do this the way I have recommended but I can't understand the death grip some people have on a toilet plunger with a hose fitting." -JimN