First off, I am not a lawyer nor am good at this kind of thing, but you may want to be careful with that, though.
If we take an analogy, it is like you have made a nice violin pattern that goes really well with, say, "Sunrise" by Norah Jones. You are free to record the violin pattern only and distribute it for free, of course, but you cannot include "Sunrise" to your record because it is copyrighted material and people are supposed to pay to own the record of the song.
I see the same problem with distributing CPanel's function(s).
However, I may be wrong. The best place, though, to ask this question is at
http://www.cpanel.net/ they have a very good web community there.
Hope this gives you some idea. I personally think GPL is a noble thing. Keep it up!