Upfont License

Hello!

I was reading on the website that WPMUDEV are under the GPL, and I just wanted to confirm this.
I am creating a Multisite, and I would like to use the Upfront themes to provide end users with a website builder.
I would like to double check that the license allows me to charge end-users for using Upfront, or the the product must be free. In addition, does the license allow me to make changes to the Upfront themes, and can I white-label?

I appreciate your help.