I want to develop and distribute a GUI (Graphical User Interface) for an existing command-line tool (LibLouis). I want to do so using .NET WPF or Windows Forms, but the library of Liblouis is written in C++ and I do not have the possibility to merge C# and C++ as far as I know. The library was licensed under the LGPLv2.1+, so that would have been nice, if I could use it.
Unlikely, I had to decide to use the command-line tools, which are licensed under the GNU General Purpose License (GPLv3+) license.
My question now is: can I create a GUI for those command-line tools and sell this GUI under my own propretary license?