If you haven't already, try uninstalling the standard Gadgetbridge app before installing the bangl.js flavor! (Like @andiohn suggested) That did it for me.
I had the same problem with no install before doing that.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
If you haven't already, try uninstalling the standard Gadgetbridge app before installing the bangl.js flavor! (Like @andiohn suggested) That did it for me.
I had the same problem with no install before doing that.