If you have the keystore file already, it can be as simple as adding a few parameters to your build command:
./gradlew assembleRelease \
-Pandroid.injected.signing.store.file=$KEYFILE \
-Pandroid.injected.signing.store.password=$STORE_PASSWORD \
-Pandroid.injected.signing.key.alias=$KEY_ALIAS \
-Pandroid.injected.signing.key.password=$KEY_PASSWORD
No permanent changes to your Android project necessary.
Source: http://www.tinmith.net/wayne/blog/2014/08/gradle-sign-command-line.htm