Earlier this year Microsoft confirmed that they are bringing GATT Server APIs support to Windows 10 Mobile in an update which is coming later this year.
The GATT Server profile is important to enable notification support for Fitbit’s wearables, who use the profile to send text message and other notification alerts.
The latest update regarding this feature comes from Microsoft staffer Clint Rutkas, who posted at the end of September on UserVoice:
The Bluetooth team is hard at work adding GATT Server APIs to Windows and we expect GATT Server should be in the Windows Insider Preview builds and SDK later this year. When APIs get added in to the OS and SDK, any developer can start updating their app. Once the next release of Windows is available, any app that want to use these new features will be able submit an update to the Store.
Developers like Fitbit have been waiting for years to get GATT server support on Windows. Once Microsoft adds this support later this year, developers like Fitbit can update their apps to support SMS/Call notifications on wearable devices.
It is likely however that we would have to wait for the official rollout of Redstone 2 early next year for support to actually come to applications.