A big difference with the others (many) static site generators, at least with most powerful, is that they often use framework like Bootstrap or have themes. Of course, g.static doesn't use them. You can certainly apply them to site, but it's not my main goal to have a nice site done in some seconds: it's to have a site dinamically updatable. Sites I'm working are simple and specialized, and due the fact I've not a lot of free time, I have to do a virtue from necessity, saving time.
For example, the use of Bootstrap generallyis an help to build responsive sites, but I want to leave to webmaster the control, because I got a lot of automatically created pages, that could simply enter in a Bootstrap world, but need a software chain more complex than now.
Of course, there is the possibility to create a theme by saving files like css, templates and chunks, packing them and putting in the appropriate directories.