reset robot location in rviz

edit

You can delete the model and respawn it. Have a look at how to spawn model here.

You can also use python script to do it for your. In gazebo_ros_pkgs package there is spawn_model python script that takes care of it (it's the same one used with roslaunch command).

You can spawn model using python like this:

from gazebo_msgs.srv import SpawnModel

def spawn():
    req = SpawnModelRequest()
    req.model_name = "model"
    req.model_xml = model_xml
    req.robot_namespace = "namespace"
    req.initial_pose.position.x = 0.0
    req.initial_pose.position.y = 0.0
    req.initial_pose.position.z = 0.0
    req.initial_pose.orientation.x = 0.0
    req.initial_pose.orientation.y = 0.0
    req.initial_pose.orientation.z = 0.0
    req.initial_pose.orientation.w = 1.0
    req.reference_frame = ''

    try:
        srv = ServiceProxy('/gazebo/spawn_sdf_model', SpawnModel)
        resp = srv(req)
    except ServiceException, e:
        print("   Service call failed: %s" % e)
        sys.exit(1)

    if resp.success:
        print(resp.status_message)
        return 0
    else:
        print(resp.status_message)
        return 1

Where model_xml is your urdf or sdf file.

Then you can delete models like this:

def delete():
    try:
        srv = ServiceProxy('/gazebo/delete_model', DeleteModel)
        req = DeleteModelRequest()
        req.model_name = "model"
        resp = srv(req)
    except ServiceException, e:
        print("Service call failed: %s" % e)
        sys.exit(2)

    if resp.success:
        print(resp.status_message)
        return 0
    else:
        print(resp.status_message)
        return 1

So if you want you can create your own python script which takes care of it. Using roslaunch command is fine as well, but you're limited sometimes.