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This is what I see in my Ubuntu 14.04:

~$ dpkg -s ros-indigo-gazebo
ros-indigo-gazebo4-msgs
ros-indigo-gazebo4-plugins
ros-indigo-gazebo4-prerelease-msgs
ros-indigo-gazebo4-prerelease-plugins
ros-indigo-gazebo4-prerelease-ros
ros-indigo-gazebo4-prerelease-ros-control
ros-indigo-gazebo4-prerelease-ros-pkgs
ros-indigo-gazebo4-ros
ros-indigo-gazebo4-ros-control
ros-indigo-gazebo4-ros-pkgs
ros-indigo-gazebo-msgs
ros-indigo-gazebo-plugins
ros-indigo-gazebo-ros
ros-indigo-gazebo-ros-control
ros-indigo-gazebo-ros-pkgs

So it seems there are some indigo-gazebo4 packages. Did you try that (like apt-get remove ros-indigo-gazebo-* && apt-get install ros-indigo-gazebo4-*)?

I don't wanna try and hassle my own system, though, so sorry for I haven't tried that before posting the answer.