How do I configure git send-email to work with gmail?

I'd like to contribute to glibc (the GNU C Library, not to be confused with GNOME's GLib library), for instance, to update the glibc/benchtests/bench-malloc-thread.c malloc speed test benchtest, but they are old and archaic and living in the Dark Ages still where we have to submit patches through email at the command-line instead of opening a Pull Request through GitHub or GitLab. :)

See glibc's contribution instructions here: https://sourceware.org/glibc/wiki/Contribution%20checklist:

• Create one or more patches using git format-patch
• Use git send-email --to=libc-alpha@sourceware.org to send the patch to the list, using --cc to get the attention of specific maintainers if needed.
• Ping at least weekly if your patch has not been reviewed yet

I've tried reading and following a bunch of resources, but none seem to be complete, namely:

1. git: 'send-email' is not a git command. See 'git --help'
2. https://coderwall.com/p/qcsiew/setting-up-and-using-git-send-email-with-gmail
3. https://coderwall.com/p/dp-gka/setting-up-git-send-email-with-gmail
4. https://git-scm.com/docs/git-send-email#_use_gmail_as_the_smtp_server
5. https://git-scm.com/docs/git-credential
6. https://gist.github.com/jasonkarns/4354421
7. https://gist.github.com/winksaville/dd69a860d0d05298d1945ceff048ce46
8. Added 8 Apr. 2023, per @mcepl in this now-deleted answer: [also incomplete] https://git-send-email.io/

Note: another project you'll need to use git send-email for in order to contribute patches is Buildroot (see here: https://buildroot.org/downloads/manual/manual.html#submitting-patches), which is used to build custom embedded Linux distributions. So, if trying to figure this out for Buildroot as well, refer to my answer.