Unlike electroless finishes being an electrolytic process an electric current required to plate the gold onto the board. The thickness of the gold coating can be easily controlled by the plating current and plating time. In the days of old edge connectors were regularly specified as requiring 7.5 micros of gold plating, P&M Services still normally plates 3 microns,  but if the thickness if left unspecified the norm within our industry is much less. To plate an edge connector a panel is normally masked with tape of peelable resist then edge dipped into the plating solution.

Hard gold can also be used as a total circuit board finish, in which case the cheapest production route can be to photo print a negative image, plate the remaining exposed copper with gold, then strip off the photo resist and use the gold as an etch resist.