Hi!

Consult the manual, "(guix) X Window".  There is an example near the end:

--8<---------------cut here---------------start------------->8---
          (define bepo-evdev
            "Section \"InputClass\"
                  Identifier \"evdev keyboard catchall\"
                  Driver \"evdev\"
                  MatchIsKeyboard \"on\"
                  Option \"xkb_layout\" \"fr\"
                  Option \"xkb_variant\" \"bepo\"
          EndSection")

          (operating-system
            ...
            (services
              (modify-services %desktop-services
                (slim-service-type config =>
                  (slim-configuration
                    (inherit config)
                    (startx (xorg-start-command
                             #:configuration-file
                             (xorg-configuration-file
                               #:extra-config
                               (list bepo-evdev)))))))))
--8<---------------cut here---------------end--------------->8---

The above example only details keyboard configuration.  For your trackpad, have
a look at libinput's manpage.
Also https://wiki.archlinux.org/index.php/Libinput may give you a good coverage
of the topic.

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