I've been bugged by this as well (see
https://lists.gnu.org/archive/html/help-guix/2018-09/msg00028.html).  There are
a few options beside Ricardo's suggestion:

- You can start the Guix daemon with --gc-keep-outputs (see "(guix) Invoking
  guix-daemon" in the manual).  This will never delete anything that is still
  required, but this leads to an ever-growing store.  If you've got limited
  storage space, you might not want this.

- The Emacs mode guix.el has a nice interface for listing store items, in
  particular dead items.  You can sort item by size, check the number of
  referrers, etc.  This is much, much faster than you bash script and should
  suit your needs most of the time.  If you don't use Emacs, you can have still
  run it in batch mode or simply re-use the Scheme code to write your own Guile
  script.

Hope that helps!

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