What Is GPL ?
WordPress Is an Open Source Software And it is Developed Upon GPL License Most people are familiar with the term “Open Source”. It means you can Modify Code, Redistribute it, Study The code , Free to use Unlimited Times or Improve the Code.
But It Doesn’t mean that developers can’t charge for the Plugin/Theme they have developed. What it means is Once you buy the Plugin/Theme from developer you are free to do whatever you would like to.