Most high-sec Gas Sites are Nebulas, which are quiet, undefended areas that have several gas clouds that can be harvested with specialized, expensive equipment. Sadly, most high-sec gasses are not worth much ISK, either, meaning that most new explorers should avoid these sites. Gas Sites are more lucrative in lower-security space, which you likely won’t venture into at first.

Occasionally, a Gas Site may be a Lab or other facility that pirates have set up to harvest the gas for themselves. These areas are a combination of Combat Site and Data Site, and contain hackable structures that are guarded by NPC opponents.

Gas sites in K-Space yield gas that is used to make Boosters/Drugs.

Gas sites in WH-space yield gas that is used in the manufacture of T3 components.

See Gas Cloud Mining for more details.