Firstly its not gold that is being used, Satellites(nowadays) are covered with Metallized kapton polyimide film.

Generally it is something called MLI (multi-layer insulation, aka thermal blanket)

these are made up of many layers of aluminized polyimide. Polyimide is a type of plastic, and yellow in color. It's coated with aluminum on the inside surface so it looks gold. Bare aluminum reflects radiation very well, which means it absorbs heat very slowly. But it also radiates heat VERY slowly. An aluminum foil sitting in vacuum in direct sunlight will heat up to a pretty high temperature. Plastic film with aluminum backing absorbs heat a little faster, but it also radiates heat a lot faster, so it works better as the outer-most layer of the MLI.