How can I publish Twist messages to /gazebo node(amcl_demo.launch)

edit

Hello! I'm runnning TurtleBot on rviz, and I want to publish Twist messages to the "/gazebo" node in order to control robot's velocity.

I wrote a publisher as below but it didn't work.
How can I solve this ploblem? Could you tell me please?

my code↓↓

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
def callback(data):
    pub=rospy.Publisher("/gazebo",Twist,queue_size=10)
    vel_msg=Twist()
    print(data.linear.x,data.angular.z)
    vel_msg.linear.x=0
    vel_msg.angular.z=0
    pub.publish(vel_msg)

def main():
    rospy.init_node("GetVelocity")
    sub=rospy.Subscriber("/mobile_base/commands/velocity",Twist,callback)
    rospy.spin()

if __name__== "__main__":
    try:
        main()
    except rospy.ROSInterruptException:
        pass

rqt_graph Image↓↓(while running "amcl_demo.launch"&"rviz"&"turtlebot_world.launch"&"above code")
rqt_graph(red arrow in this picture shows what I want to do)