Where do all these people take the gasoline?

There is literally an oil refinery in the movie.

What do you mean where do they take it?

Gastown exists to produce gasoline. The Citadel is where food is grown and water is collected. The Bullet Farm is where ammunition is produced. The three communities coexist via trade. The Citadel and the Bullet Farm rely on fuel from Gastown, just as Gastown relies on food from the Citadel, and so forth.

That's why I could never take seriously those Mad Max movies and Waterworld etc.

When it's obviously barbarian societies. With no food and gasoline resources. And those people have been in that barbarian stage for a long time. Which means they do not go to school, no one teaches them something. And you have to be smart and have knowledge how to make gasoline, how to repare things.

Yet all bad and good guys have unlimited source of gasoline to ride their adventures. No one ever eats and needs eating.
No one usually needs to drink water in the freaking desert.

While we saw in many apocalyptic shows that gasoline is the main problem people have when everything collapses. And eventually when show is long enough then we see how in few years characters start riding horses because of lack of gasoline. But in Mad Max they have working cars and bikes decades after collapse.

Do you mean where do they get it?