According to the latest docs for Qt 5.5, Gstreamer 1.0 is supported via Qt Multimedia. However, I can't seem to find much more information about it (the guy from this post seems to be in the same boat). I need 1.0 support in Windows so I am currently using QtGstreamer.
Previously in Phonon, Qt had gstreamer 0.10 support (at least on Linux). Does this 1.0 support extend to more platforms, like Windows?
GStreamer is only used by Qt Multimedia on Unix platforms. See this page for more information.
On that same page, you'll see the Windows support is limited to DirectShow or Media Foundation.