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From what I understand about Gadgetbrige, it doesn't use so many BLE services. It rather uses a Serial interface service for passing JSON encoded data packets. I am not sure about that but there is chances that the serial service used could be NUS

Thanks - yes, that's spot on. You just need the Nordic UART service. Info on what Gadgetbridge actually sends is in https://www.espruino.com/Gadgetbridge