To be more precise, GPL code/library can be linked with other code/application using whatever license. The GPL only targets -*distribution*- of such applications. In other words, if you pick a proprietary application and a GPL extension module, you are perfectly allowed to use them together as long as you keep it for yourself and don't distribute it.
OK, but this is rarely the case (not in this one actually). Since as it's not distributed we (the general public) never know...