How to deploy fresh Grails 3 app on Heroku?

Préconditions:

• create an account on heroku.com
• having one free dyno not already created (two dyno is the maximum for a free heroku account currently)
• install heroku CLI
• install sdkman/grails3

Create a new grails default app and create a file named "Profile":

$ grails create-app myapp

$ cd myapp

$ touch Procfile

"Procfile" is important for heroku plateform to configure the way the app will be launched on the plateform, copy paste this in Procfile:

web: cd build ; java -Dgrails.env=prod -jar ../build/server/webapp-runner-*.jar --expand-war --port $PORT libs/*.war

Then open the project inside your favorite IDE and edit build.gradle by adding this at the end of the existing file:

task stage() {
    dependsOn clean, war
}
tasks.stage.doLast() {
    delete fileTree(dir: "build/distributions")
    delete fileTree(dir: "build/assetCompile")
    delete fileTree(dir: "build/distributions")
    delete fileTree(dir: "build/libs", exclude: "*.war")
}
war.mustRunAfter clean

task copyToLib(type: Copy) {
    into "$buildDir/server"
    from(configurations.compile) {
        include "webapp-runner*"
    }
}

stage.dependsOn(copyToLib)

and also add dependency

compile 'com.github.jsimone:webapp-runner:8.5.11.3'

in the dependency section.
This dependency is for heroku to launch the app/

Then the git part:

$ git init
Initialized empty Git repository in .git/
$ git add .
$ git commit -m "first comit"
Created initial commit 5df2d09: first commit
 44 files changed, 8393 insertions(+), 0 deletions(-)
 create mode 100644 README
 create mode 100644 Procfile
 create mode 100644 app/controllers/source_file
...

Then create the heroku remote repository:

$ heroku create
$ git remote -v
heroku https://git.heroku.com/thawing-inlet-61413.git (fetch)
heroku https://git.heroku.com/thawing-inlet-61413.git (push)
$ git push heroku master

Initializing repository, done. updating 'refs/heads/master'

check the result by using

$ heroku open

Sources:
https://devcenter.heroku.com/articles/git#prerequisites-install-git-and-the-heroku-cli
https://devcenter.heroku.com/articles/deploying-gradle-apps-on-heroku