I know that they’re mainly composed of gas but is there any solid land underneath that or is it just gas? And if that’s the case how are they capable of having so much gravity?
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They are mostly gas. They may have small solid cores, but the vast majority of their mass is gas.
Gas has mass (what a blast!) like anything else. And therefore gas exerts a gravitational pull, too. Gas isn’t as heavy per unit volume as, say, solid rock, but the gas giants are very big, so they have a lot more volume (and therefore a lot more total mass).
Actually, though, you sort of have it backwards: the fact that they have high gravity is *why* they’re gas giants in the first place. Small planets like Earth don’t have strong enough gravity to hold on to hydrogen and helium, at least not at Earthlike temperatures. So they escape into space, and Earth ends up with a relatively light atmosphere. But the large cores of the gas giants *were* able to hold on to hydrogen and helium, and once they got started, they could hold on to more and more and grew quickly.
(As an aside, your “etc” is unnecessary: Uranus and Neptune are no longer classified as gas giants. Instead, they’re made of heavier molecules like ammonia, methane, and water ice. Today, they’re classified as *ice* giants, although “ice” here just means “a light molecule with a relatively high melting point” – they’re still gases, it’s just that the gases they’re made of are [in astronomical language] called ‘ices’.)
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