The following from
Debian Wiki worked for me with a Bluetooth Low Energy mouse (microsoft arc):
[!--quoteo(post=0:date=:name=wiki.debian.org/BluetoothUser)--][div class=\'quotetop\']QUOTE(wiki.debian.org/BluetoothUser)[/div][div class=\'quotemain\'][!--quotec--]Using bluetoothctl
If bluetooth-agent is not available, try bluetoothctl:
Start the bluetoothctl interactive command. Enter "help" to get a list of available commands.
Turn the power to the controller on by entering "power on". It is off by default.
Enter "devices" to get the MAC Address of the device with which to pair.
Enter device discovery mode with "scan on" command if device is not yet on the list.
Turn the agent on with "agent on".
Enter "pair MAC Address" to do the pairing (tab completion works).
If using a device without a PIN, one may need to manually trust the device before it can reconnect successfully. Enter "trust MAC Address" to do so.
Finally, use "connect MAC address" to establish a connection.[/quote]
After pairing it once, the connection is almost instantly when turning on the mouse, even after rebooting Debian with no further actions.
edit: using gemini x27 4G version