Sure you can! You just can’t distribute the application without also distributing its entire source code under the GPL license.
It is completely fine to use it to build an application for a client. Such an application cannot become a product without becoming open source, however.
So you need to change your application into a GPL licensed application. That’s what I meant.
GPL is a great license, but for libraries (or „building blocks“) it greatly limits their usage. Not everybody wants to license their application as GPL. Just like not everybody likes bananas. Some prefer peaches or oranges.
That’s fair, but it is a part of the license. To me, it makes sense that collaborative open source projects are licensed under the GPL: it forbids taking fruit from the community garden for free and selling it for profit.
But for a library project, GPL is quite an unusual choice. I don't know any commonly used library, that has such a restrictive license. Most of them use LGPL, Apache, MIT, BSD or something similar. Otherwise your library is commonly doomed to be a stillbirth.
Ghostscript is one example, a lot of users ran into legal trouble using it.
