That only depends on what you want, which kind of scene you try to create. You can place any kind of props everywhere. There are no limitations. So, if you want to place the MoonGate in a wood, it would be good to add some trees around it, and so on.
But it also depends on your camera angle. If you view the person in the gate from a lower angle, it may happen that you can only see the sky in the background. Therefore just add a sky.
And it also depends on the mood. For example if the scene is set in the night, there is not much light and you can see not much from the environment.
And it also depends on if you are setting up a real scene or a fantasy scene. In a fantasy scene you could just surround it with some colors.
So you see, the possibilities are almost endless, always depending on what you want to achieve. I always found it heplful to watch images and to get some inspiration from them.