Follow up:
I just realized that gsettings isn’t installed by default. I installed it on the user profile not the system.
Looks like I got gsettings from the glib package.
wegei8@guixbtw ~/guix-config$ ls -al /home/wegei8/.guix-home/profile/bin/gsettings
lrwxrwxrwx 1 root root 73 Dec 31 1969 /home/wegei8/.guix-home/profile/bin/gsettings -> /gnu/store/m94abzcc5q8vgbw4sqy54c6dshkf56sj-glib-2.83.3-bin/bin/gsettings
Looking at guix search, it looks like there are multiple outputs to the glib package. I installed :bin.
$ guix search glib
name: glib
version: 2.83.3
outputs:
+ doc: documentation
+ static: static libraries
+ bin: executable programs and scripts
+ debug: debug information
+ out: everything else
Again, in my case, I installed on the user profile and then ran the commands above. This is how you can install the bin output of glib via CLI.
$ guix install glib:bin
OR, if you’re using guix home.
(use-modules (gnu home)
(gnu packages glib))
(home-environment
(packages (list `(,glib "bin"))))
I don’t really understand the syntax, but it works. 
`(,glib "bin")
Quasiquote, something, something. I should probably read more about Scheme…
Uh, also, wait. There’s one more thing I want to mention! If you don’t want/need gsettings all the time, you don’t even have to install it! You can just do this!
guix shell glib:bin
Then you run whatever gsettings command you want and exit the shell when you’re done! No need to keep it around.
(Although, there are reasons you may want to permanently install glib:bin (on the user profile)…)