What do they mean when they say Jupiter is a "gas" planet? Could a rocket be shot through it? Could an astronaut (or spacecraft) "land" on it?

Jupiter is called a gas planet, because it is largely composed of hydrogen (about 75% by weight, 90% by actual particle count) and helium, with only a tiny fraction being anything heavier than those.

Hydrogen and helium are gases, at least under conditions close to what we humans encounter on our tiny home called Earth, so astronomers decided to be pragmatic (or “down to Earth”), and started calling planets like this Gas Giants. (Gas giant - Wikipedia).

Due to its size, Jupiter is relatively close to being a star … albeit a tiny one (Brown dwarf - Wikipedia). Not large enough to start fusion in its interior, but still very impressive. And through the gravitational pull of its mass creating a pressure large enough to overcome the atomic bond of molecular hydrogen, creating a weird substance called metallic hydrogen (Metallic hydrogen - Wikipedia), which is a liquid!

Only in its outer layers the hydrogen is still a gas, forming a thick atmosphere. A very small fraction of the atmosphere is made up of compounds such as ammonia, sulfur, methane, and water vapor, which explains the dizzying array of white, grey, brown, yellow and red …

Trying to ‘land’ on Jupiter would be easy - just aim for the planet with your spaceship, and let gravity do the rest. You will get to see one of the most amazing cloudscapes known to man from up close:

It would be so awesome! I would love to come with you … but I won’t.

After enjoying the view for quite a while (as it takes really long to get down to the clouds), you will be caught up in the pretty swirls, which I expect will initially look like you are surrounded by a colorful, dense fog. But that won’t last long, as plunging down deeper it will get pretty dark, and the temperature of the clouds will quickly rise to 1,340 °F (725 °C). Not surprising, as after all you are touching what is almost a star.

Please make sure to send your photographs, video, and spoken impressions back to Earth before you melt, will you? Thanks you!

Enjoy your trip!