Firstly, what version of Android are you running on your Galaxy S·II? Garmin Connect Mobile app for Android requires Android‑4.0 or later, and according to Wikipedia, Samsung began rolling out an update to Android 4.1.2 "Jelly Bean" for the device in February 2013. If you've kept your device's firmware up to date, then you should be able to install the Garmin Connect Mobile app on it.
Secondly, also according to Wikipedia, the Galaxy S·II is limited to Bluetooth‑3.0 using the last Android version released by Samsung (4.1.2); Bluetooth‑4.0 support has been introduced in Android 4.3 versions, however the upgrade to an alternative firmware is required. As far as I'm aware, ">‑Energy">Bluetooth Low‑Energy mode requires Bluetooth‑4.0, so even if you can install and run Garmin Connect Mobile on your Galaxy S·II, you still won't be able to sync your watch with your handset, but only view data that has already been otherwise uploaded to Garmin Connect (e.g. via Garmin Express).
If you're frustrated that your handset fails to keep up with your changing requirements (including syncing with recent model peripherals), then perhaps it's time for you to upgrade that component of your personal IT landscape. It's not up to manufacturers of everything else to endeavour to make their products backward-compatible with what you have, so that you don't have to spend more money (albeit not necessarily on their products) to keep what you use day to day fit for purpose.
