I don't think that the 21 million Bitcoins are related in any way to the amount of Gold in the world.

My guess is the following:

Satoshi had to make a design for Bitcoin that made it future proof, even in case Bitcoin would be used worldwide by many people. So the total amount of units are 21 million Bitcoins, with 100 million Satoshies per Bitcoin. These are plenty of units, in case they are needed to satisfy demand.

Why now 21 million  Bitcoins?

Maybe he just liked the number 21 for some reason, like his girlfriend was 21 or so...or something similar.

And 21 is 3 x 7 .  7 is a popular number in the West:

"As an early prime number in the series of positive integers, the number seven has greatly symbolic associations in religion, mythology, superstition and philosophy. The seven Classical planets resulted in seven being the number of days in a week.[1] 7 is often considered lucky in Western culture and is often seen as highly symbolic."

https://en.wikipedia.org/wiki/7