So what kind of gas can I fill my house with to keep the heat on the lower levels? I hate to heat the attic for no reason.

I am not sure you want to fill your house with anything else than air. You need to breathe somehow. You may instead want to isolate your house.

I would have totally thought you would have said the equator is the main factor and where the mountain is located.

The location of the mountain also plays a role, but those are not amongst the main factors. I am not sure to understand the equator part of your sentence.