I do believe it can be used for free, with an occassional pop-up to register if you like it.
It serves the purpose of being able to create an entire local pipeline, quickly using an interface to do so. Setting up project workspaces with livereload, Sass, Babel, Uglify, etc.
Well worth the $20, imo - since you may be spending more time trying to find a temporary solution, for other projects eventually, as well.
As mentioned, Gulp is my current approach, since it makes no sense to me when this becomes a requirement inside of the actual application. If it has to do with compiling assets, that should be done outside of what will eventually serve up the results of what was created for public consumption, imo.