I solved my issue by customizing my authentication email templates (saved to the Gatsby project “/static/admin/” folder where the Netlify CMS lives) following this post.
For noobs like me, it is worth pointing out that when you point Netlify to the location of these custom email templates in Settings->Identity->Emails, you drop the static folder, so the location for me was: “/admin/invitation.html”
For Netlify folks looking at this, please update the NetlifyCMS instructions for Gatsby. These instructions do not work if you’re not using Identity for sitewide login (and only want login to NetlifyCMS for yourself/small team for a personal website). Given that this is a known issue with email confirmation setup, the quickstart steps should direct changes to custom email templates to target the /admin/ site in the common case where Identity is not working for the site as a whole, OR streamline installation in a way that obviates this set of steps.
Also, it would be useful to at least link out to other posts that provide a bit more detail about setting up logins and how the CMS system should behave (I take it basically the same way on the server and local side) would help lower the hurdles for those just getting started with Netlify, Gatsby, Hugo, etc. The current instructions are too brief IMO.