I have followed all of your steps and while my Code 100 pairs to my device, when I open the Marshall Gateway it never shows the code connected, nor does it give me the option inside the app to connect. I have uninstalled and unpaired multiple times with the same result.
If you drag the menu button across, click on the Code tab at the top, this will give the option to scan for amps within the amp, have you updated your firmware on the code unit to help with connection, also could you please confirm which mobile device you are trying to connect.
Swiping and tapping woke it up and I connected, menu on iPhone works fine now. Thanks.