This slogan was on postmarks used in South Africa about 40 years ago. There are still lots of Krugerrands around to start the ball rolling on a world currency metal.

http://goldkrugerrandferret.com/about-krugerrands/how-much-krugerrand-worth/

We would still need something to serve the need to buy things worth less than say a current US10 dollars. Gold coins with 1/200 of a troy ounce of gold are pretty small!

That's one reason most countries had circulating silver coins as well as gold.