include git-versioner into your project

1-global_build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.0'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

// Optional: configure the versioner
/*ext.gitVersioner = [
        defaultBranch           : "develop",  // default "master"
        yearFactor              : 1200,       // default "1000", increasing every 8.57h
        snapshotEnabled         : false,      // default false, the "-SNAPSHOT" postfix
        localChangesCountEnabled: false       // default false, the (<commitCount>) before -SNAPSHOT
]*/

// import the script which runs the version generation
apply from: 'https://raw.githubusercontent.com/passsy/gradle-GitVersioner/master/git-versioner.gradle'

2-app_build.gradle

android {
    defaultConfig {
        ...
        buildConfigField 'String', 'REVISION', "\"$gitVersionName\""
    }
    
    productFlavors {
        ...
        beta {
            applicationIdSuffix '.beta'
            versionCode gitVersion.version
            versionName gitVersionName
        }
        
        playStore {
            versionCode 21
            versionName '3.0.1'
        }
    }
}