gradle.properties

Put this into ~/.gradle/gradle.properties

RELEASE_STORE_FILE={path to your keystore}
RELEASE_STORE_PASSWORD=*****
RELEASE_KEY_ALIAS=*****
RELEASE_KEY_PASSWORD=*****


Modify your build.gradle like this:

...    
signingConfigs {

   release {
       storeFile file(RELEASE_STORE_FILE)
       storePassword RELEASE_STORE_PASSWORD
       keyAlias RELEASE_KEY_ALIAS
       keyPassword RELEASE_KEY_PASSWORD
   }
}

buildTypes {
        release {
            signingConfig signingConfigs.release
        }
}
....
Then you can run gradle assembleRelease

http://stackoverflow.com/questions/18328730/how-to-create-a-release-signed-apk-file-using-gradle