For this you can use the spawn_model script from the gazebo_ros package. Assuming you have Gazebo already opened and "my_robo" is a package in your catkin workspace, you can use a script similar to this:

<?xml version="1.0"?>
  <launch>

    <param name="robot_description" command="cat '$(find my_robo)/urdf/robot.urdf'" />

    <node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-param /robot_description -urdf -model robot" />
  </launch>