The full texts: GPL-3.0 and the Skelet Plugin Exception.
Open source, with room for your business
Skelet is GPL-3.0, so Skelet itself stays open. What you build on it is yours, under any license you choose.
Can I run my store on it?
Yes, for free.
Can I build client sites on it and charge for them?
Yes.
Do I have to publish my plugin or storefront?
No. Plugins, remote providers, storefronts, dashboard extensions, language packs and themes are covered by the Skelet Plugin Exception. License them however you like, closed included.
So what does the GPL ask of me?
Only this: if you change Skelet's own code and give that changed Skelet to someone, they get its source under the same license. That's what stops a closed fork.
Does importing the SDK make my code GPL?
Not by itself.
Can I use the Skelet name and logo?
They aren't part of the code license, so ask first.