Don’t want to piss on anyone’s parade but where is my calculation wrong?
Gold weighs about 19.3 grams per cubic centimeter. That means if you had a small gold dice cube of size 1 cm x 1 cm x 1 cm, (i.e., with a volume of 1 cubic centimeter), it would weigh 19.3 grams. So 1 grams would theoretically fill a volume of 1/19.3 cubic centimeters or 0.050 cubic centimeters.
Volume is area x height, (i.e., area x thickness). So theoretically 1 grams can plate an area x thickness totaling 0.050 cubic centimeters. If the thickness is 20 microns, i.e., 0.0020 cm, then the area you can plate with that much gold is 0.050/0.002 = 25 square centimeters.
3g of gold would plate a 20 micron 75 sq cm... I would guess that a two tones need a lot less.