Despite your hope, there is not even any inherent symbolism; gravity is simply a coincidence.
I'm interested in the base game but hear a lot about the "essential" but out-of-print expansion. Is it really essential? Do folks still enjoy the base game without Civilizations? Thoughts?
I'd put it this way: if there was no expansion, the base game would be great and well worth owning with excellent replayability. Does the expansion add good stuff? Absolutely.