How to create a slope/ramp in Gazebo editor

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The easiest way is to create a box in SDF, and give it an angle. For example:

<?xml version="1.0" ?>
<sdf version="1.4">
  <world name="default">
    <include>
      <uri>model://ground_plane</uri>
    </include>
    <include>
      <uri>model://sun</uri>
    </include>
    <model name="box">
      <static>true</static>

      <pose>0 0 0.2 0 0.4 0</pose>
      <link name="link">
        <collision name="collision">
          <geometry>
            <box>
              <size>1 0.5 .1</size>
            </box>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <box>
              <size>1 0.5 .1</size>
            </box>
          </geometry>
        </visual>
      </link>
    </model>
  </world>
</sdf>