Note that there is no difference between a ROS workspace and a catkin workspace. The layout for a workspace is typically <PATH_TO_ROOT>/src/<PACKAGES>. Choose <PATH_TO_ROOT> how you see fit, put all packages in the src folder below (in your case, the qt_gui_core package). You can have multiple packages in src.
Before you compile, go to the src directory and call catkin_init_workspace, remember to call source /opt/ros/<DISTRO>/setup.bash (where <DISTRO> is indigo, jade, kinetic, whatever you have).
You don't need to call cmake manually, catkin_make will do that.
After you compiled, call source <PATH_TO_ROOT>/devel/setup.bash after compilation to actually use this workspace.