Twenty Fifteen Premium Theme [closed]

You might want to look at what the GPL License entails. This Wikipedia article does a pretty good job of explaining:https://en.wikipedia.org/wiki/GNU_General_Public_License

“Any licensee who adheres to the terms and conditions is given permission to modify the work, as well as to copy and redistribute the work or any derivative version. The licensee is allowed to charge a fee for this service, or do this free of charge.”

Hope that helps