How did an early understanding of gases lead to a better understand of the atom

Gas molecules are more spread apart. Actually A LOT more spread apart. Liquid molecules are compacted, and solids are very compacted. To understand an atom, it's best to observe it "unaffected" by other atoms. This means when two atoms are very very far apart (such as gas, a very low pressure high volume) the molecule/atoms act as if they are in an isolated system. This way they can judge all sorts of stuff from an atom without feeling impact of the effect by other molecules. There's a lot more stuff involved such as physical chemistry, where you need to isolate the atom in order to measure its potential, dissociationg energy ... blah blah (simulation with particle in a box, rigid rotor, harmonic oscillator, quantum mechanics). the list goes on forever.

The early understanding of gases led to the understanding of the atom, because Dalton derived the first theory of the atom through experiments related to a mixture of gases.

Explanation:

Dalton's greatest legacy as a scientist, however, was the development of the first atomic theory. It was through various experiments related to gas mixing and knowledge of the works proposed by Lavoisier that Dalton's atomic theory emerged in 1808. Without the early knowledge of gases, Lavoisier's works would not have been established and Dalton would not be able to establish the basis for understanding the atom.

Dalton's atomic theory was based on experiments, but none of these experiments managed to reveal the atom clearly. So Dalton called the atom the smallest part of matter.