How to appease GDM so I can login with another window manager?

[Kyle Andrews]

Dear Guix,

I am using herbstluftwm as my window manager on my old Guix laptop. It
shows up as an option in a bottom right corner of GDM. On my new laptop,
this menu doesn't show up. In both cases I have included herbstluftwm as
a required package in my config.scm file. My old laptop's config.scm
makes no mention of herbstluftwm in the configuration anywhere else than
in bringing in the package. However, I further know that this package
does create an xsession output to a herbstluftwm.desktop file under
/gnu/store.

Does anyone have any ideas what the missing incantation is to make
herbstluftwm (or any other relatively niche window manager with a
.desktop file) show up on the GDM menu?

Thanks,
Kyle