system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does it mean when it is said that a planet is made of gas?

Like, how is that possible? Is actual gas? Can I get to the other side of it just going through it? And more importantly, how the hell does all of that mass stay together if it's gas? Wasn't is supposed to just spread and try to occupy the maximum volume possible?
gpt
Answering your questions in order

1. It means that the planet is made of gas

2. Gravity

3. Mostly, but if the planet gets big enough, the core becomes a super-critical fluid, something not quite gas, not quite liquid.

4. Yes, if you can withstand the pressure, fluid density, and can push your way out of the gravity well.

5. Gravity again. 

6. Yes, and the maximum size is the size of the planet, because gravity keeps it there.

While it may seem like gravity only attracts small objects to big ones, everything is attracted by everything elses gravity, so if you start with a relatively small iron core that goes through a gas cloud, the attractive effect compounds, gathering more and more into itself, as long as there is gas to collect.