Update: The software update is out, and you should now see grippers onboard the robot and in Gazebo simulation by default. Should you want the URDF for just the grippers, they can be configured and constructed dynamically by Xacro with the baxter_description packages:
1) edit baxter_description/urdf/left_end_effector.urdf.xacro to have your desired end effector configuration
2) construct it with xacro:
$ rosrun xacro xacro.py --inorder `rospack find baxter_description`/urdf/left_end_effector.urdf.xacro
Old Answer: I've been working on this for the upcoming release-1.2 of Baxter software. We have not released the update for onboard the robot, but I expect we will do so early next week. I will update when the software is available.
If you would like to test Baxter with grippers in the Gazebo in the mean time, check out these installation instructions:
http://sdk.rethinkrobotics.com/wiki/S...
I need the URDF for my Gazebo simulation. I have the environment configured but I don't have the URDF file with the grippers attached. I have an experimental file but when I try to control the grippers with the keyboard controller, I get a message that says it "cannot be used with custom grippers."