The Gear Fit, like Samsung's Gear smartwatches, is officially only compatible with the company's smartphones, a limitation that has been one of the most criticized aspects of Samsung's wearables. However, just like the Galaxy Gear was found to work with the Nexus 5 and other non-Samsung devices, the Gear Fit has been discovered to be similarly compatible. The folks over at asdasd and 9to5Google were able to make the Gear Fit work with the Nexus 5 and the HTC One/HTC One M8, by sideloading the necessary apps on the device and then pairing the Gear Fit through Bluetooth.
As expected, while most functions work correctly, some of them don't, such as the quick reply options and alarm notifications. Also, it might be necessary that the device you're trying to pair it with will need to be running at least Android 4.3, which is the minimum OS version required for Samsung's fitness tracker and smartwatches. It's a pretty straightforward process to pair the Gear Fit with the aforementioned devices, and you can check out the whole procedure at the source links below, or take a look at the video for a visual walkthrough.