You’d need a plugin that would validate email addresses against that gem. A simple one would do it just when it tried to create the account. A more sophisticated version would do it before it was submitted. I’d guess it’s 1-3 hours work for a seasoned discourse c developer.
See Developing Discourse Plugins - Part 1 - Create a basic plugin or marketplace. There’s also a all-the-plugins repo in the github discourse hierarchy that’s a good place to look for examples.