I followed the instructions on your README, but no matter how many times I install the webapp generator, yo fails to recognize it. I run yo gulp-webapp and get:
You don't seem to have a generator with the name gulp-webapp installed.
I've hit an issue like this in the past. This SO question (and my response to it) might help you out.
My fix was much simpler:
npm install --force -g generator-gulp-webapp