Hello Dmitriy Sidin,
Unfortunately, UWP does not support GATT over classic Bluetooth. This is a limitation of Microsoft's UWP Bluetooth APIs. The reason why UWP can't do this is because UWP's BluetoothLEDevice
class only works with BLE devices and UWP's BluetoothDevice
class only handles classic Bluetooth but provides no GATT access. Overall, Microsoft designed UWP with strict separation between these two protocols.
However, you can try using RFCOMM instead If your device supports both GATT and RFCOMM. You can read here for more detail about RFCOMM
Thank you!