You can buy a Gear VR in the US now, but you need a Galaxy Note 4 to get any use out of the device. While the headset comes with extra sensors, much of the computing power takes place on the smartphone side of things. An update is now rolling out to the Sprint and Verizon versions of the Note 4 that get things ready to go.
In the Sprint update, version NK2, that's all there is to see. Verizon's changelog continues on for a tad longer. Build number N910VVRU1ANJ5 comes with updated Galaxy Essential and Amazon widgets. ISIS wallet has also had its name changed to Softcard. These changes are joined by a lengthy list of bug fixes.
You can read up on both updates at the source links below.