Who owns my grids and output?

Your grids are the product of your own work, and therefore they are yours to do with as you like. We do ask a small fee for the service of housing and generating your grids, but that does not mean we own the output. There is no license for using the CSS, JavaScript, PNGs or Sass output, because it is yours.

From our Terms & Conditions:
“It may be that technically, the grids you create may or may not be protectable as copyright either under English law or your local law, wherever you happen to be. But regardless of that, we confirm that we regard the grids that you create as yours, and we will not make use of them save as you direct when using Gridset or in order to demonstrate for marketing or promotional purposes the sort of thing Gridset can do.”

Go right ahead and use your grid ouput for whatever. Templates for CMSs, demos and github repos are all fine uses for your grids, so long as you agree to distribute them in this way.