When gold coin collectors and investors talk about a twenty dollar gold coin, they are usually referring to a historical or currency coin, used at one time as actual money, in the USA.
This historical twenty dollar gold coin is the double eagle, issued between 1850 and 1933. This is a large coin, about 34mm in diameter, weighing about 33 grams, and having an actual gold weight of about one ounce (actually 0.9677oz).
Over the period this coin was issued by the various US Mints, its design changed. From 1850 to 1907, the coin is called the 'Coronet Head' or the 'Liberty', as the obverse features a crowned female head and stars, with the word 'Liberty' on the crown and the date of issue below, while the reverse features the eagle and the value of the coin – 'Twenty D.' in the earlier coins, then 'Twenty Dollars' in the later ones. There were also some other some minor changes to this design over the years, but it is a remarkably consistent coin in appearance over the date range.
An 1899 example of the Liberty or Coronet Head twenty dollars gold coin of the USA
The redesign of the coin was done by Augustus Saint-Gaudens, and the twenty dollar double eagle cold coins of 1907-1933 are known as 'Saint-Gaudens'. The obverse of this coin features the famous walking Liberty with torch and olive branch, while the reverse has the value and an eagle flying at sunrise.
A 1922 example of the Saint-Gaudens twenty dollar gold coin
The Saint-Gaudens design was reused recently for the modern bullion gold eagles issued by the US Mint.
Buying a twenty dollar gold coin
There are available here: