To reproduce:

#include <QtCore/QCoreApplication>
#include <QDebug>

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
qDebug() << "Hello World!";
return a.exec();
}

Run from the debugger in Qt Creator with MinGW. The output from qDebug() is not visible in any window, including the Application Output window or debug views of gdb.