The term definitely predates D&D - the term "twenty dollar gold piece" has been in use for the $20 Double Eagle and $10 Eagle coins of the late 19th century, and also the $5 gold coin, as well.

"Gold Piece" In Print

The term is used in the Lebanon Daily News, 1 Nov 1965, of Lebanon, Pennsylvania, bottom, in an advert for old coins under the left column of text (to the right of the comics)

Four gold pieces: One (1) $20.00 gold piece, two (2) $10.00 gold pieces and one (1) $2.50 gold piece.

This alone establishes the phrase "gold piece" for gold coins in routine use prior to D&D. But let us press a little further back... say, 1913? Here's a quote from the 5 August 1913 Cedar Rapids Evening Gazette, from Cedar Rapids, Iowa, page 4, top of the third column:

The five cent piece ls the day laborer of our coinage. It la the hardest working and most successful bit at money In use In these United States. The twenty dollar gold piece Is very popular and is madly sought after In the best society; the five dollar bill has millions of friends and the hard silver dollar can be found nestling In the pocket of almost every man. But none of these like the five cent piece.

We thus have established a pattern of use for gold coins of being called "gold pieces" in the press, spanning over 5 decades; clearly not a D&D origin; not even viably a wargaming origin, for 1913 is the year of the first printing of H. G. Wells' Little Wars, the first commercially released set of wargaming rules in book form.

Searching Project Gutenberg, several ebooks have it in use...

These without clear denomination prefixed:

• Pinocchio (1883, Tr. ??? )
  Author: Carlo Collodi, 1826-1890
  Translator: Carol Della Chiesa, 1887-
• The Younger Set (1907)
  Author: Robert W. Chambers
• A Drama on the Seashore
  Author: Honore de Balzac (1799-1850)
  Translator: Katharine Prescott Wormeley (1830-1908)
• Tiger Cat (1938)
  Author: David H. Keller
• Pâkia (1901)
  Author: Louis Becke

And several with clear denomination in dollars:

• The Corner House Girls in a Play – How they rehearsed, how they acted, and what the play brought in
  Authors: Grace Brooks Hill, R. Emmett Owen
• The Young Bank Messenger (1898)
  Author: Horatio Alger, Jr
  

Piece

Piece is, according to several dictionaries, a common term for coins in general, of whatever denomination is specified. The quote below is excerpted from the etymology online page:

piece
early 13c., "fixed amount, measure, portion," from O.Fr. piece (11c.), from V.L. *pettia, probably from Gaulish (cf. Welsh peth "thing," Breton pez "piece"), from O.Celt. base *pett-.
[...]
Piece of Eight is the old name for the Spanish dollar (c.1600) of the value of 8 reals.

Commentary

It's pretty clear that it's a generic term for a gold coin, and for several US gold coins as well. In the US, it seems to be predominantly the popular $5 coin of the late 19th and early 20th centuries, but can be used collectively for the $2.50, $5.00, $10.00 and $20.00 gold coins; The silver coins of similar values were $0.10, $0.25, $0.50, and $1.00. Note that, still to date, "2 bits" is $0.25... a reference to the not uncommon practice of breaking Pieces of Eight (Dollares, or Reals) into 8 "bits" of an eighth-dollare each... I suspect that this is the origin of the 20:1 Silver:Gold ratio in AD&D...

That's interesting, as being a non-native English speaker I always assumed it was one of the accepted meanings. So as every time I realize one of these things, let's check the Wiktionary entry for soak:

Verb

soak (third-person singular simple present soaks, present participle soaking, simple past and past participle soaked)

4. (transitive) To allow (especially a liquid) to be absorbed; to take in, receive. (usually + up)

   I soaked up all the knowledge I could at university.

So yes, even though it is not the most often used meaning, "to soak" can be used for "to absorb", and not only for liquids. So "soaking damage" for "receiving damage and absorbing it" is not a pure invention, it is accepted - though unusual - English language.