<C-h C-i> (info-lookup-symbol) is a good way to find documentation
on C functions while editing (see Info Documentation Lookup).
The GMP manual can be included in such lookups by putting the following in your .emacs,
(eval-after-load "info-look"
'(let ((mode-value (assoc 'c-mode (assoc 'symbol info-lookup-alist))))
(setcar (nthcdr 3 mode-value)
(cons '("(gmp)Function Index" nil "^ -.* " "\\>")
(nth 3 mode-value)))))