Yes you can use .gjs files for route templates in Discourse right now. In fact, we just converted a whole load of them today in core! Check out https://github.com/discourse/discourse/tree/main/app/assets/javascripts/discourse/app/templates for examples.
“someone” is Discourse - we sponsored the development of ember-route-template, and it’s in our GitHub organisation 
We’re not on the latest Ember 6.x yet, but we use that addon to enable the functionality.