I think that HG verbal kids probably do ok. As you said, there is always something at their level to read in the classroom, library, or home. However, a PG gifted verbal kid who is no longer satisfied with appropriate level reading material but that perhaps would like to create some of his/her own (writing a book, poem etc. may have a hard time getting a teacher to teach him/her the tools they need to do so. I just think there is a clearer path for math than there is for literature.
I am not so sure.
GT kids can go get books from the library. Look things up. We are steeped in language. Its very accessible.
GT math kids may not be able to process higher math books due to language issues. The math roadmap is not obvious to the uninitiated and the material is vanishingly small in volume compared to most other material.
Most math textbooks outside of upper division college today have a very high noise/signal ratio. I don't think they are appropriate for GT kids. I hated them.
And doing math problems is a much different process from reading a book.
IMHO, a condensed approach stressing Logic and Real Analysis which picks up Geometry, Algebra, Number Theory, Discrete Math, and Calculus along the way might be more appropriate for GT kids who are already very good at arithemetic and who can think conceptually. You could also briefly touch upon other math and non-math advanced subjects along the way.
