Steps to reproduce / test case:

Compile gcleak.cpp file and run the attached source code.

In an terminal run xrestop.
Observe the number of GC allocated for the test application in xrestop.
The reported value will quickly rise when resizing the window of the test application.

Constructor of QStylePainter calls QPainter::begin() which results in call to XCreateGC.
The created GC is not freed when QStylePainter goes out of scope.