To spawn the Stretch in gazebo's default empty world run the following command in your terminal.
roslaunch stretch_gazebo gazebo.launch
This will bringup the robot in the gazebo simulation similar to the image shown below.
In gazebo, you are able to spawn Stretch in various worlds. First, source the gazebo world files by running the following command in a terminal
echo "source /usr/share/gazebo/setup.sh"
Then using the world argument, you can spawn the stretch in the willowgarage world by running the following
roslaunch stretch_gazebo gazebo.launch world:=worlds/willowgarage.world