There are two types of gas: Amaranthine and Vespene.
For each type, there are three different forms: diluted, refined and concentrated.
Vespene: Found on comets (max depth 50), planetoids (max depth 150), and planets (max depth 250). Appears green.
Amaranthine: Found on asteroids (max depth 100) and moons (max depth 200). Appears pink.
Gas can be diluted, refined, or concentrated. Diluted gas can be used to make refined gas, which in turn can be used to make concentrated gas.
Gas originates from geysers. The closer to a geyser, the more concentrated the gas mined will be. This can be seen: diluted gas appears as faint puffs, while concentrated gas appears as large puffs with saturated color. Digging directly into a geyser, however, causes an explosion that may kill the astro-bot.