As the rulebooks do not give an actual answer to this, the best I can give you is an approximation.

What we know:

Throughout D&D rules across editions, a gold coin has  most frequently been described as being the about same size as a US Half Dollar Coin. This citation comes from either a verbal description ('about an inch across.' ref: p212 of 4E PHB), or from an 'exact size' picture (ref: p168 on 3.5E PHB). So, for simplicity's sake, I am going to use the thickness of a half-dollar coin (is thick enough to be sturdy) and the 'inch across' quotation and image.

A quick check on Wikipedia tells me that a Half Dollar Coin is 0.085 inches thick. This gives us a volume of 0.067 cubic inches.

Given that pure gold weighs 0.7lbs per cubic inch, we can compute that a half-dollar sized coin should weigh 0.0469 lbs.

In Basic D&D and AD&D 1E, a gold coin was described as weighing a tenth of a pound. Which is...frankly...too heavy. The coin would have to be nearly twice as thick in order to give you that much heft.

However, from AD&D 2E onward, the weight of a gold coin has consistently been held to be one third of an ounce, or 0.0208lbs. (ref: 5E PHB p143, 4E PHB p212, 3.5E PHB p112). Thus, we could reasonably assume that either the coins are smaller than advertised, or they are not pure gold.

Historically, copper has been used as a common material to alloy gold with for making coinage. Copper weighs 0.324lbs per cubic inch, which would give us a pure copper coin weighing 0.0217 lbs....which is still too heavy. So it can't be a copper alloy. Tin is another option, and it weighs 0.204 lbs per cubic inch. A pure tin coin of this size would give us a mass of 0.0137 lbs. Tin is the lightest material that gold is commonly alloyed with that would be available in the middle ages, so we can guess that the gold is alloyed with tin.

So, to compute this, we have two equations as a system...given that x is the percentage of gold and y is the percentage of tin.

$$x + y =1$$

Representing that the percentages must add up to 100%

$$x*0.0469 + y*0.0137 = 0.0208$$

Representing that the percent of gold and tin must add up to 1/3 of an ounce.

Solving this system of equations gives us the following values

$$x = .2139$$
$$y = .7861$$

Which translates to a coin that is 21.39% gold and 78.61% tin.

This translates to 5K Gold...a quite low gold content. With a gold/tin mix, I'm not sure this would even still look like gold.

There's a complication to this worth mentioning. According to 5E PHB p157 and 3.5E PHB p112: 1lb of gold is worth 50gp. And weighing a third of an ounce, 50 gold pieces equals one pound. Historically, including during the middle ages, money was worth the value of the metal it was made of. A 50lb hunk of gold, or 50lbs of gold coins was worth the exact same thing. This was the international trade standard for much of civilized history...money was worth the metal it was printed on. Before there was a big international economy that determined relative currency values...if you changed the precious metal content of your coins, you changed the value of your coin relative to another nation's.

Thus, it is distinctly possible that the coins are 'pure gold' and thus their value is maintained by medieval standards. And the mismatch in weight and/or size is simply because the designers didn't bother with the math, and set the weight based on a gameplay decision rather than on realism. 

And if you're concerned about difficulty....D&D is set in a society that has reliable and sturdy steel production. Refining 'pure gold' is much easier, and was accomplished far earlier in history.