I recently built a custom map, but whenever I try to test the map I get a gray floor. I can see the mountains and the sky. I am able to zoom in, but nothing else? I followed a map editing tutorial to the letter and I am pretty flummoxed as to what I should try or need to do next.

You did not select an aircraft for the player. If the player does not have an assigned aircraft when you play the mission you get what you described.