Air insulated substation means air acts as the dielectric medium but in case of gas insulated substation SF6 is the dielectric medium. Gas insulated substations are having more advantages than air insulated substations.

Gas insulated substations are in use in places where a normal, above ground substation would constitute an eye sore for the community. A gas (SF6) insulated substation may have underground feed lines and be nearly invisible to the average person -- except for the location where the transmission feed wires go underground from the transmission towers. distribution circuits supplied by gas insulated substations normally have an alternate feed from another substation, to allow maintenance work on the gas insulated site.

sulphur hexa-fluouride

Air insulated means the substation is in the open air, and the dielectric strength of the air is used to isolate conductors from each other and from ground. Gas insulated means a specific gas is used for insulation; this gas is usually an inert gas such as nitrogen. Gas insulated substations, by definition, are required to be inside a contained structure, or large piping must be placed around all conductors to hold the gas.

This seems like a question from an electrical course, and is probably best answered by your course materials.

It's your test question, not ours, and there won't always be someone to ask for the answer. Earn your diploma.