You will need an empty SQL or Mongo database before running the setup process, so go ahead and fire up a new database using your favourite database service.
Gravity uses Mailgun for sending transaction emails to users, so go ahead and register a free Mailgun account and set it up. Please refer to the Gravity Server Installation instructions for more information.
To run your app on the Android simulator, you'll need to download a free copy of Android Studio. Publishing your app on the Google Play store will require that you register as a developer with a one-time fee of $25.