Boldbeast Call Recorder for Android records calls clearly and loudly in two directions in your GT-I9100G.
If the system of your GT-I9100G is Android 2.X, no need to root, just install and go.
If the system of your GT-I9100G is Android 4.X, you should root your phone, and
- enable the "Recording Tune" option if you are using the Common version.
- enable the "Change Audio Controls" option if you are using the Creative version.