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’.)
It’s mostly a gradient. Earth atmosphere and earth itself have a very obvious line separating them, mostly because they’re not the same material. With gas giants, the deeper down you go, the thicker the gas gets, to the point you could consider it solid down far enough, but it wouldn’t happen at a discrete line, just gradually. They’re still very dense close to the middle, so gravity is still there.
The moon with it’s thousands of craters suggests that gas planets will have some rocks at the center due to their enormous gravity fields.
>I know that they’re mainly composed of gas but is there any solid land underneath that
It’s weirder than that. The gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune) may have a small rocky core deep inside them, but before you get there are layers of supercritical fluids that are neither gases nor liquids, layers of hydrogen under such incredible pressure that they behave like a liquid metal (for the gas giants), or layers of bizarre forms of ice that are solid at temperatures of thousands of degrees (for the ice giants).
There’s no solid surface you could really stand on: you would be crushed and incinerated long before you reached any surface that could support your weight.
>If that’s the case how are they capable of having so much gravity?
There’s a lot of gas. I mean, a *lot* of gas. Just the very outermost layers of Jupiter’s atmosphere weigh as much as the whole Earth.
In short, one really knows. For example, we’ve sent probes to enter Jupiter’s atmosphere, but none have survived most of the way through the atmosphere due to intense heat and pressure.
They likely do have a rocky/icy core though. Jupiter has an atmosphere that transitions from frozen gas crystals to gas to gas in a supercritical state (not really a gas nor liquid) to liquid gas and finally, most likely, a rocky core.
They have gravity because they’re made up of a lot of particles attracted to each other, each with their own mass and somewhat close to each other. Everything with mass has gravity.
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