Yes, the Samsung Gear S2 is compatible with a lot of android devices.
The general compability requirements are:
Gear S2 pairs with smartphones running on Android 4.4 and higher with over 1.5GB RAM.
Have a look at Samsungs official device-compatibility website.
Also quoting the Review of the Gear S2 by Techradar here:
In the past Tizen has only been compatible with Samsung smartphones, greatly limiting their potential user base. Thankfully, the Gear S2 is compatible with any Android phone running 4.4 and higher with over 1.5GB RAM. I tested it using a Moto X Style and HTC One M8, neither of which caused any problems.
Although almost all features are compatible with any Android phone, Samsung Pay is only available with Samsung devices. So if you want to pay for your groceries with your watch, you better get a Galaxy or Note (or an Apple Watch and iPhone).
Samsung has also hinted the watch could work with iPhone eventually, but don't hold your breath.