I believe you could only use this library if the whole resulting app is also GPL.

You can work around that. If you have an application that also has a command-line option, then you could create a UI for that application with this library. The UI module for your application will have to be under GPL because this library is licensed under GPL. But your core program can still be closed-source, as long as you distribute both parts separately. A bit convoluted, but I've seen some applications that do this - they don't distribute the GPL code directly, but ask you to download it separately. As far as my understanding of the license goes, this conforms to the GPL only if (1) the GPL code isn't essential or a core part of the application, and (2) you don't distribute both as one application (which implies that the GPL part is absolutely necessary part of your application, thus violating the first rule).