I came across this promising looking GUI editor for Bootstrap (http://getbootstrap.com) called Bootstrap Studio. I bought a copy a few weeks and have been playing around with it to prototype some designs and it works pretty good. The creators of it have been releasing software updates every couple weeks based on user feedback which is helpful. They have a free online demo you can use as well. Feel free to check it out https://bootstrapstudio.io I am not involved in anyway with Bootstrap Studio or its creators