how to insert a force or torque sensor to gazebo model？

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Hi Skhan, sorry for the late reply, one way to do this is to create a model plugin, for example, see this tutorial on how to create a model plugin. In the plugin update loop, you can get the force torque information from one of the joints in the model by calling:

    physics::JointWrench wrench = this->model->GetJoint("...your model joint name...")->GetForceTorque(0u);

On the SDF side, in your model, you must set <provide_feedback> to true for this to work.