3.15 Emacs

C-h C-i (info-lookup-symbol) is a good way to find documentation on C functions while editing (see Info Documentation Lookup in The Emacs Editor).

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)))))