I was having the same issue. I found that using the "LIBS" variable, instead of using:
"CONFIG += link_pkgconfig
PKGCONFIG += ..."
worked for me. So you put "LIBS += -lQt5GStreamer-1.0 -lQt5GStreamerUi-1.0" and whatever other libraries you need. If the library files are in an unusual place, you can use "LIBS += -L" followed by the directory. I'm not sure if it matters, but I'm using QMake version 3.0 and Qt Creator 4.0.0.