The expanding Gorilla Glue I'm aware of is something I only use on porous surfaces. On hard plastics it wants to either push the pieces apart, or squeeze out if well-clamped. Its operational principle is forcefully penetrating porous cellular spaces to get a grip. Perhaps there is another non-expanding version.
If it is just cracked and not broken off at the stem below to the switch, super glue or Krazy glue is an option I'd try. But then I'd also consider, depending on the degree of damage, Loctite's or Permatex's plastic bonder epoxies. They come in "5 minute" and "20 minute" varieties. The longer-curing one can result in a smoother finish as it has time to self level more. But I'm talking about using it only if super glue between the parts doesn't hold, and many times it won't on plastics. The epoxies will have to essentially cover the outside of the knob to tie the parts together after sanding to roughen surfaces for a good epoxy grip; then you'd have to Dremel-form that surface to clean it up to an original-looking shape and paint it.
By the time you do all that you may wish you'd just got a new switch, but plastic epoxies have fixed a lot of stuff for me lately.
Joel