[ROS1, Gazebo] URDF doesn't work on Gazebo

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Hi everyone, I'm quietly new to Gazebo, and I need to spawn my robot in it. I wrote the URDF file by hand, and it works fine in MATLAB, and also in Rviz, where I can actuate the joints without any issue. By the way, trying to import it in Gazebo, it just appears as a model (on the left, through the true structure of the system), but it doesn't spawn in the simulation. I tried to launch it both with the "manual" method (launching an empty world before and then spwaning the robot) and with a complete launch file. Can anyone help me?

<?xml version="1.0"?>

<robot name="my_robot" xmlns:xacro="http://ros.org/wiki/xacro">

   <material name="red">
          <color rgba="1.0 0 0 1.0"/>
      </material>



<material name="qbmove/flange">
    <color rgba="0.0 0.5 0.8 1.0"/>
  </material>

  <material name="qbmove/cube">
    <color rgba="0.35 0.35 0.35 1.0"/>
  </material>

  <material name="qbmove/shaft">
    <color rgba="0.8 0.8 0.8 1.0"/>
  </material>

  <material name="qbmove/default">
    <color rgba="0.35 0.35 0.35 1.0"/>
  </material>

  <material name="SoftBlue">
    <color rgba="0.0 0.0 0.8 0.9"/>
  </material>

  <material name="SoftLightBlue">
    <color rgba="0.0 0.4 1.0 1.0"/>
  </material>

  <material name="wheel">
    <color rgba="0.50 0.50 0.50 1.0"/>
  </material>


  <link name="base_link">
    <inertial>
      <mass value="0.00000001"/>
      <inertia ixx="0.00000001" ixy="0" ixz="0" iyy="0.00000001" iyz="0" izz="0.00000001"/>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </inertial>
  </link>

  <joint name="rot_x" type="revolute">
    <axis xyz="1 0 0"/>
    <limit effort="50.0" lower="0" upper="+1.57075" velocity="50.0"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <parent link="base_link"/>
    <child link="body"/>
  </joint>

  <link name="body">
    <visual>
      <material name="SoftBlue"/>
      <geometry>
        <cylinder radius="0.04" length="0.74"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0.248 0.50"/>
    </visual>
    <inertial>
      <origin xyz="0 0.248 0.50"/>
      <mass value="12.787"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="2.33" iyz="0" izz="2.33"/>
    </inertial>
    <collision>
      <geometry>
        <cylinder radius="0.04" length="0.74"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0.248 0.50"/>
    </collision>
  </link>
  <gazebo reference="body">
    <material>Gazebo/Blue</material>    
  </gazebo> 

  <link name="upper_axle">
    <visual>
      <material name="SoftBlue"/>
      <geometry>
        <cylinder radius="0.02" length="0.368"/>
      </geometry>
      <origin rpy="-1.57075 0 0" xyz="0 0 0"/>
    </visual>
    <inertial>
      <mass value="1e-8"/>
      <inertia ixx="1e-8" ixy="0" ixz="0" iyy="1e-8" iyz="0" izz ...