I have followed the steps described by Hosain in this link and I am already developing an Android application fully in Groovy. No Java. Ou Yeah!
If you want some context, almost a year ago @cedricchampeau introduced at GR8Conf how to use Groovy on Android. I watched both Developing Android Application with Groovy and Android and Groovy, a winning pair and read the related blog post. I recommend you both videos to get answers to most common questions. How much system resources do I sacrifice to run Groovy on Android? Is there a performance penalty cost? Spoiler: around 1Mb and no performance penalty with @CompileStatic
Problems with using Groovy on Android? To my understanding it is not possible to use popular dependency injection frameworks such as Dagger.