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Now you're moving the goalpost. Questions about how, what, when and where things happen are a different class of questions from why they happen. The first set deals with the mechanics of things in a purely objective context, whereas questions about why tend to imply a subjective context linked to conscious intention, and therefore the question, "Why does gravity exist?" is a loaded question because it assumes that there was some conscious intent in bringing gravity into existence, and that in-turn implies a creator who had a purpose for it. 

I won't go as far as Dawkins and say that "why type questions" are "silly". Sometimes we can answer them in terms or cause and effect, but unless the context is clear, questions about why, particularly in these kinds of discussions can constitute an informal fallacy, and when they do, they may become irrelevant within the context that the discussion was initiated, which is why I started out by saying you're moving the goalpost. If you want to move out of the how we know things and into the whys of existence, then that's a whole different discussion.

Here's another video that might help you get a grip on basic astronomy. In it you'll find explanations for such things as how the planets, including gas giants form:

How Planets Work

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