Enlightenment is a window manager. You can look at the project's web page here
"enabling enlightenment in gnome" would probably be someone's method of saying "replace your current window manager with enlightenment". Gnome is a desktop environment, and can work with pretty much any window manager. It's simply a matter of personal taste which window manager you use, and then taking whatever steps you need to tell Gnome which window manager to use when Gnome starts up.