How to open the PR2 gripper?

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PR2 gripper from Gazebo model library has a joint for controlling how wide should be the gap between finger. Unfortunately I can't find a reliable way of controlling that joint. Ideally I want to set it's position instantaniously. I tried to do that from a custom plugin with the jointController->SetJointPosition method, but I get the following error:

[Dbg] [Link.cc:1481] we have a loop! cannot find nice subset of connected links, this link r_gripper_r_finger_link connects back to parent r_gripper_r_finger_tip_link.
[Wrn] [Joint.cc:820] failed to find a clean set of connected links, i.e. this joint is inside a loop, cannot SetPosition kinematically.

I also tried to use jointController->SetTargetPosition, the gripper fingers moves somewhat, but I can't work out PID values that would work OK.