Since August 2016, the structure of Gitter Rooms and Communities have changed considerably as we've started to allow for the creation of communities and rooms that aren't directly related to objects in GitHub
When creating a new community, you can choose to link the community to an organisation or repository on GitHub. If you link the community to an organisation, this means that all members of the organisation on GitHub will be admins of your community. You can then easily create any room in the community, or create rooms that are integrated with repositories owned by that organisation. This will allow features such as the activity feed in the room to automatically receive webhooks of activity in the repository on GitHub and link issues in the room to issues on GitHub.