License violation

[Barry Nelson]

You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:

a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

Modidying Filezilla to create Filezilla Pro and adding features and then charging a required subscription fee violates the terms of the GPL license.

[Barry Nelson]

Source code release for the Pro version is required to comply with the terms of the license.

[Tim Kosse]

The GPL only applies to software you have received under the terms of the GPL.

FileZilla Pro on the App Store is not distributed under the GPL, as agreed to by the copyright holders.

[Barry Nelson]

Any software which includes a component licensed under the GPL or copyrighted undertake GPL must abide by it's terms. Both FileZilla and FileZilla Server as downloadable from filezilla-project.org are free open-source software distributed under the Terms and Conditions of the GNU General Public License (GPL) version 2 or (at your option) any later version. All software derived from GPL-licensed code must in turn be licensed under the GPL.

[Tim Kosse]

This only applies to code you have received under the GPL. The copyright holders of that code however can still distribute this code, or binaries created from the code they own, under a different license.

This use-case is explicitly covered in the GPL FAQ: ​https://www.gnu.org/licenses/gpl-faq.en.html#HeardOtherLicense

I heard that someone got a copy of a GPLed program under another license. Is this possible? (#HeardOtherLicense)

    The GNU GPL does not give users permission to attach other licenses to the program. But the copyright holder for a program can release it under several different licenses in parallel. One of them may be the GNU GPL.

    The license that comes in your copy, assuming it was put in by the copyright holder and that you got the copy legitimately, is the license that applies to your copy.