Thank you very much, now it works fine :-)
Here is the solution:
ui->gauge_quick->setSource(QUrl(QStringLiteral("qrc:/qml/QtQuickProgressBar.qml")));
ui->gauge_quick->show();
auto rootObject = ui->gauge_quick->rootObject();
connect(this, SIGNAL(setGauge(QVariant)), rootObject, SLOT(setvalue(QVariant)));
emit setGauge(15.5);