gravity does not work on my model

edit

Hi guys, I'm having a slight problem with Gazebo 11. I have created models with Fusion360 and imported them as stl into gazebo. However the gravity tab under "links" is False and it doesn't let me enable it to True... Is there anything I need to do within my model sdf ? Admittedly, I did not use the most clean method to generate my files. I simply copied my stl into another object's folder and renamed all the sdf files and sort. Can anyone help me? Thanks