relations of gz topic & rostopic, and how to check the information of a gz topic message type(like: mav_msgs::msgs::CommandMotorSpeed)?

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I have learned that gz topics use almost the same subscribe and publish mechanism with ros topics, but what confuses me is, one name(gazebo/command/motor_speed) of two topics appears in a project. One is a ros topic and the other is a gz topic.

I checked the information of the two topics.

The first one has message type of : mav_msgs/Actuators. The latter one has message type of : mav_msgs::msgs::CommandMotorSpeed.

-1. I want to know the difference between gz topics and ros topics, which matters a lot as for things I am learning now.

-2. How can I check the content of the message type mav_msgs::msgs::CommandMotorSpeed(or any other gz topic message types)?

I would be really grateful if one can help.