I'm not familiar with the GT2 watch or its companion app, but what you are trying to do isn't possible within Android. Having said that, a solution may be possible in the Huawei wear app that connects to the watch.
Is there an option to disconnect the phone from within the app? If so you can use UI intergation to replicate the keypresses. It will probably take some experimentation to get the macro to work. I would recommend toggling the connect /disconnect button in the app, so that the macro can turn connection on after leaving the car.
This approach will only work with the screen on.
I would suggest having a manual method of running the macro to disconnect GT2 before you get to the car. Then when the car connects the GT2 has already been disconnected.
The car Bluetooth disconnected could be the trigger to run the macro to turn on GT2 Bluetooth connection.