A gas insulated substation is a substation in which a gas (specifically, sulfur hexafluoride, or SF6) is used as the insulating medium rather than air. Gas insulated substations require a much smaller footprint than conventional air insulated substations, and therefore are commonly used in inner city applications. They can also be built inside buildings which makes them popular in applications where the outdoor environment is harsh such as deserts.
Gas insulated transformers are transformers that use SF5 gas as the insulating medium. Standard large power transformers use mineral oil as the insulating medium - but mineral oil is flammable and cannot be used inside structures or in other situations where fire hazard is an issue (such as offshore platforms, either oil pumping stations or gathering stations for offshore windfarms). .
So both gas insulated substations and gas insulated transformers are ideal for special applications where space and flammability are concerns. They are not universal replacements for air insulated substations or oil-filled transformers because they are far more expensive, and in the case of substations, gas insulated substations are far more difficult to modify or expand than air-insulated installations.