Start with Glib, its quite easy and well described here: Glib Reference Manual
GObject is mostly a lot of boilerplate code to achieve object orientation. Try out the “maman bar” examples found in the GObject Reference Manual
For me, GStreamer took the most time to understand. The documentation is good but there is alot to read (GStreamer docs). After reading about the basics, try starting a pipeline from the cmdline using gst-launch. Then read the full manual about application development and do some examples. Before trying to build your own plugin, check out some simple plugin and try understand the different parts, a simple place to start is the identity plugin
Good luck!
/Joel Larsson