> 99% of the difficulties of a web design is in supporting all possibile client screen sizes / ratios.
For a web designer.
For a normal person, 99% of the difficulties of web design is that they need to get a PhD in HTML, CSS, & JavaScript, in order to make some text and images show up. PowerPoint/Publisher are usable by anyone. The Web today is basically an employment scheme.
> I'm not aware of any GUI for creating responsive content
If all you want is to show someone some text and pictures, that should be (is, actually) incredibly simple.
If you want to show someone "responsive content", that is a different use case. It's a custom program at that point. So just ship them a custom program. It used to be common, trivial actually, to mock up a program in a visual IDE and send it to someone. The program would be small and fast and networked. We used a thing called Visual Basic, and nearly anyone with the most basic programming skills could use it. Worked great.
Today we do the same thing, only they are called "mobile apps" and are written in Java for a dedicated mobile operating system. Just like years ago, these are proprietary platforms run by quasi-monopolies, so you have to make your program multiple times to send it to everyone. But we could improve on that.