1- Copy the model you want to use in .gazebo/models directory.
or
2- Gazebo -> Insert ->
or
gazebo example.world
Most models come with maps.
export GAZEBO_MODEL_PATH=/home/<user_name>/.gazebo/models/small_house/models/
gazebo small_house.world
export GAZEBO_MODEL_PATH=/home/<user_name>/.gazebo/models/office/models/
gazebo office.world
export GAZEBO_MODEL_PATH=/home/<user_name>/.gazebo/models/bookstore/models/
gazebo bookstore.world
export GAZEBO_MODEL_PATH=/home/<user_name>/.gazebo/models/hospital/models/
gazebo hospital.world
gazebo hospital_two_floors.world
export GAZEBO_MODEL_PATH=/home/<user_name>/.gazebo/models/factory/models/
gazebo factory.model
export GAZEBO_MODEL_PATH=/home/<user_name>/experiment_rooms/models/
cd experiment_rooms/worlds/room1
gazebo world_dynamic.model