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You can try to add some friction on your joint to make them resist the gravity force.

You may need to control your joint with a ros tool or a gazebo plugin to make them keep a particular position.

Maybe this will give you some hints : http://answers.gazebosim.org/question/13029/why-does-the-robot-links-fall-downwards-when-gravity-is-disabled-and-no-controllers-are-used/

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