GitHub recently introduced significant changes to how Organisations interact with the API. This new feature means that Organisations can block API Applications and access needs to be requested or enabled. You can read more about this change here.
We request API access during signup so for existing users creating or joining new Organisations is fairly problematic as the request to grant access to the Organisation for the API application will not occur again.
There are one of two things you can do:
1. Disable the setting.
If you visit
https://github.com/organizations/YOURORG/settings/oauth_application_policy you can disable the restriction from this page.
2. Manually grant access
If you visit your own application settings and find any Gitter applications there, click on the name of application and you should see the list of Organisations that allow access to the application and you can grant access here. Please note that if you enable Gitter for access to private repositories, you will need to do this for the Gitter Private Repo Access application as well.
After activating this, you may need to wait up to 5 minutes and then refresh & reload Gitter.