Gold pieces are very much the universal currency of role-playing games. But they are not completely generic like most Dungeon Masters make them out to be.

Most countries mint their own coinage, in many countries it is illegal to use another countries currency. So those gold pieces you got out of the dragon's lair must be exchanged for local currency before they are used. Other countries (particularly those that do a lot of foreign trade), will accept gold coinage from most nations in the area.

Some nations mint no coinage at all. These countries tend to accept any and all coins, no matter what is stamped on them. This is how most AD&D campaign worlds seem to work (even if it is not realistic). These nations are about the only place you can spend that bag of gold coins you found in that 900 year old tomb. Merchants in these countries tend to weigh coins carefully for large purchases, and will often go by the weight, instead of the number of the coins.

The gold piece varies in value from game to game. But there seem to be only a few popular values for them.

The first is the strong gold piece. This is what the D&D series of games uses. The strong gold piece is worth roughly $20 USD. Most common items (like beer and bread), are purchased with smaller denominations, such as copper pieces.

The second is the weak gold piece. Very common in computer games, and some non-TSR Fantasy role-playing games. The weak gold piece is worth roughly $1 USD. In these games the gold piece is usually the only coin around (with no smaller denominations existing).

The last is the very weak gold piece. Really only used in RPG games for console systems. The very weak gold piece is so worthless that almost everything is priced at hundreds or even thousands of them.