There are no license restrictions when using GPL software. You can download and modify the source code any way you see fit and you are not forced to make available any changes to the source code.
Your are only required to make available any modified source code when you make a distribution of that GPL software. The theory is that you must make available the same freedom that was granted when the software was distributed to you. Now this in itself is not a restriction but only an issue of compliance with the GPL agreement.
Now there are some restrictions on linking and/or including closed source software or certain types of software that have an incompatible license with the GPL, but once again this only applies to a distribution and not actual end use.
