I haven't had luck with Samsung or LG phones or tablets. The tablet connects (listen to youtube audio thru the amp) but Gateway doesn't connect.
Have you tried the update on the amp, and also a reinstall of the app.
Thing that worked for me was to delete EVERY previous bluetooth connection to the other devices...maybe try that. This is my post where i explain what i did https://my.marshall.com/forum/question/2516/fix-your-software-please?filterType=recent