Can gas be broken down into simpler substances?

It depends on the gas.

If it's a pure gas--one with a single element, like oxygen or nitrogen--it's already as simple as it can get.

If it's a mixture of gases, like air is, you could feed it into a gas liquification system and collect the individual pure gases as they condense.

If it's a compound like carbon dioxide, you need to break it down into its elements and then you can separate it out. (But in the case of CO2, one of the elements is naturally a solid so you can just scrape it off the walls of the reactor.)