Gifted comes under the special education funding umbrella here, so 100 percent of the students in my children's congregated gifted classes have IEPs.
However, children with "dual exceptionalities" are a little rarer (that's kids who've been found to be both gifted and learning disabled). My son is one of those, and he's of the opinion that maybe 2 or 3 kids in each class have this designation. (Just counting off, there's a Gifted/ADD kid, and a Gifted/Autistic kid, and of course my kid...)
I have no idea what the overall percentage of IEP'd kids might be.