Filled states a fraction; plated states nothing
This is the distinction the whole page turns on. A gold-filled mark names a fraction of the item total weight: 1/20 12K GF means one twentieth of what the piece weighs is 12 carat gold. That is a quantity, so the gold content can be calculated exactly. A 20 gram item stamped 1/20 12K GF carries a one gram layer, half of which is gold, giving half a gram.
A gold-plated mark names a karat and no quantity at all. 18KGP tells you the plating is 18 carat and nothing whatsoever about how much of it there is — typically well under half a micron, but the mark does not say so. There is gold present and the stamp does not state how much, so no honest figure exists. This tool prints that rather than a zero, because zero would assert there is no gold, and that is not what the mark says either.
HGE, meaning heavy gold electroplate, is still plating despite the word. RGP, rolled gold plate, is bonded in the same way as gold filled but thinner, and states no fraction.
Real gold, and still not worth recovering
The gold in a gold-filled piece is genuinely there — bonded under heat and pressure, measured in percent rather than microns, and it wears for decades rather than months. It is also not worth extracting. Separating a bonded layer from its base metal costs more in chemistry than the recovered gold returns, which is why refiners decline gold-filled scrap outright and why a buyer who takes it is buying an object rather than metal.
Put in proportion: a 20 gram item at 1/20 12K GF holds about half a gram of gold. The same 20 grams in solid 14 carat would hold 11.7 grams, more than twenty times as much. That ratio is why the two are different markets rather than different grades of the same thing.
The exception is vermeil, which is gold over sterling silver rather than over base metal. There the core is itself precious, so the piece does have recoverable metal value — as silver, at silver prices.
What it is worth instead
Gold-filled jewellery from the late nineteenth and early twentieth centuries is often extremely well made, and much of it is collected and traded well above any metal it contains. Antique gold-filled watch chains, lockets and bangles have their own market. The question simply moves from what a piece is made of to what it is — the same question that decides whether solid gold should be melted, arrived at from the other direction.