The GPL is a license agreement that open source projects use (among others that are available – like the BSD license, LGPL, etc). The main firmware in question is called Marlin and that is released under the GPL V3 license. The TLDR on that license is that you can do the following:
- Free to use it however you want as long as source code is provided to the people using it directly or upon request
- You can even sell it as long as you provide source to the people purchasing it
- If you distribute it on a physical product then the customers of that product are entitled to the exact source used to build the firmware on the machine.
The full text is here from the GNU GPL website: https://www.gnu.org/licenses/gpl-3.0.en.html
More information will be added soon.