I just had the same problem and found a simple solution:
Just open a two terminal windows, first run gulp watch in one window, it will continue running, so just head over to the second terminal window and begin working with ionic serve
Now every time I change my ionic.app.scss file it gets compiled and the app updates accordingly.
I’m using the exact same files that the thread-creator posted, in my case I had to manually add the line “./scss/*.scss” to “watchPatterns”.