Change effective on 06 June 2017
Grails 2.x is no longer supported on Heroku (deprecation notice). Your Grails 2.x apps WILL continue to work on Heroku, but the buildpack will:
You can continue using the Grails buildpack by manually setting it with the following command:
$ heroku buildpacks:set https://github.com/heroku/heroku-buildpack-grails
Support for Grails 3.x will continue to be provided by the Gradle buildpack.
For more information on using Gradle and Grails 3.x please see the Dev Center guide for Deploying Gradle Apps on Heroku.