meta: add mailmap entry for garygsc

Add mailmap entry for garygsc and re-run AUTHORS to remove duplicate
entry.

PR-URL: https://github.com/nodejs/node/pull/39588


Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>