I wouldn't start boiling hair gel to remove humectants, LOL. To do it right you'd need a distillation apparatus. Not very practical for home use! Sally's has "Salon Care" gel, one or 2 of which don't have glycerin and have pretty decent hold, I understand. Most of the things in hair gel won't tolerate high temperatures - I don't know if they'd just break down immediately or change texture or what (hockey puck, anyone?) but I am pretty sure you wouldn't have "hair gel" when you're done.
Rats, now I'm really curious.
http://science-yhairblog.blogspot.com/