Hi Olivier,
You can find it if you decompile the APK and search for “na_client_android_vaillant”. The key will be somewhere around that.
I will send it to you by PM so you don’t have to worry to much about finding it.
The IP of the gateway is not needed : the communication is done directly to Netatmo’s API, which sends the control back to the gateway (same way as when you use the app)
Have fun !