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How do I get a gripper on the simulation model?

you'll have to create a new xacro macro that combines both the UR5 macro (I'm guessing from ros-industrial/universal_robot) and a model of your gripper. Then instantiate that macro and use the resulting urdf as a basis for your further work.

If you haven't already, I'd recommend you do the Learning URDF Step by Step tutorials, as that should give you some insight into how this all works.