> If I understand well, I installed the packages guile, guile-gcrypt and
> Emacs, obviously. :-)
> And emacs-guix comes from MELPA, Geiser too.
> Then I am a bit confused.

I strongly recommend that you install emacs-guix from Guix only, and not from
MELPA.  The thing is that Emacs-Guix relies on a scheme cache which goes out of
sync if you switch from MELPA to Guix to install the package, or vice versa.

Once you've sorted out the install, delete ~/.cache/guile/ccache, and restart Emacs.

> Last, 'guix package -s gmsh' returns the package that I am looking for.
> However, `M-x guix p n gmsh' does not.

For me, there is no "n" in `M-x guix p`.  Anyways, you are probably better off
using `M-x guix-packages-by-name`.

-- 
Pierre Neidhardt
https://ambrevar.xyz/