The gel isn't as soluable as a foam or spray, it needs more of a "you wipe/scrub stuff" touch to get things clean. I'm personally a fan of Greased Lightning, if you can't find that, next is Purple Power. But Simple Green works well also. Just so you don't have any problems, cover up the tps/fusebox/anything else that could cause some serious damage if it were to get wet, with a plastic grocery bag or something of the sort. I'm usually just careful where I spray, but if its your first cleaning and there's alot of junk under the hood chances are you're going to use alot of water and alot of cleaner, just better safe than sorry.
Mike