Adding Service Changed Characteristic to Generic Attribute Service?

I think it could be something internal to the Bluetooth stack. I guess you could see if adding a value to it made the characteristic appear?

The thing is Service Changed really shouldn't be needed. The only place I have ever seen it used is for device firmware updates, so it definitely wouldn't be anything the Garmin watch should need.

Other things I guess could be an issue:

• Connection interval - I'm not sure if you can see what connection interval the real footpod reports but it may be you can use NRF.setConnectionInterval to change Espruinos - even if it's only to make the maximum interval higher (the Garmin may well want to use a larger interval to lessen power consumption).
  
• Address - the actual Mac addresses look very different. I guess it could be the Garmin refuses to connect to a 'random' address device and a requires Public address: http://www.espruino.com/Reference#l_NRF_setAddress
  
• Pairing - it might be the Garmin needs to pair? Something like this may help http://www.espruino.com/BLE+Security#passkey-pin-pairing