GPL licenses are designed to guarantee your freedom to share and change free software:
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
What this means is that you are free to change the code as you see fit. You aren't locked in to any proprietary system, and not dependent upon any single software vendor to update the software powering your website.
For more information, see the Free Software Foundation’s website on GPL: