As reported in Sammobile, the I9100G variant of Galaxy S2 has officially received the new Android 4.1.2 Jelly Bean update. You might see a notification via OTA or Samsung KIES software, but bear in mind Samsung is rolling out this update gradually.
In this Android 4.1.2 Jelly Bean update for Galaxy S2 I9100G, you’ll experience the new UI, the Nature UX, included with the Galaxy S3,Galaxy S3 mini, Galaxy Note 2. In addition, the Project Butter software which brings a smooth performance and a more stable OS is also included.
If you are running on custom rom, you won’t get the OTA update. Hence, flashing your device with the latest Firmware is needed. If you plan to manually install the Android 4.1.2 XXLSR OTA Update on your Samsung Galaxy S2 I9100G follow the guide given here.