GStreamer Qt WINDOWS

You have to specify the gstreamer libraries against which your binaries must be linked.

According to this documentation for qmake, by issuing LIBS += -Lc:/gstreamer/1.0/x86/lib you are instructing qmake to look for libraries within the given path, but not which of them to actually link to your binaries. I am not familiar with gstreamer, so I'm not sure what libraries must be linked in the specific case you presented, but I guess you'll find them all in gstreamer/1.0/x86/lib. If unsure you could add them all to the list prepending the lower case "l" to their names. For instance, if the library were called math, you'd add it by appending -lmath to the list. Just be cautious not to add multiple versions of the same library, say a debug version and a release version, at the same time, or you most certainly will get multiple reference linking errors.

Instead of manually specifying the libraries which should be linked to your binaries as suggested above, you also have the option to use pkg-config to do the hard work for you. This documentation for gstreamer states it suficies to add the following to the .pro file:

CONFIG += link_pkgconfig
PKGCONFIG += QtGStreamer-0.10

The drawback to this approach is naturally that you have to get pkg-config to work on your system first.