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I think this problem is trickier than you first assume. Gazebo doesn't yet support deformable materials, it is a rigid body constraint solver. Thus, a belt can only be approximated in Gazebo by a chain of many links. However, URDF is generally used for describing robots and has further constraints over SDF. The important one being that SDF can support closed chains, but URDF can't. Nonetheless, creating an SDF of a belt chain riding on two cylinders may be more trouble than it's worth, if what you're trying to do is approximate a conveyer belt in Gazebo.

One solution to your trouble that avoids the above complexity is to add a static table with no friction and place your block on top. Then create a gazebo plugin or ros node that sets the velocity of the block. See section 6: Set and Get Model Pose and Twist in Simulation via service.