Nepture is not made of water; it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, and “ices” such as water, ammonia, and methane.
It is kept together by gravity, same a for the earth; all of the gas and stuff listed above is matter and all matter feels the pull of gravity. A gas giant planet is where there so much of the gas in a single place that it is pulled together to form a ball.
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