Why does gold oxidize despite being a stainless metal?

Sure, it seems like a contradiction, but there is a scientific explanation.

The gold we commonly use is not pure, most of the objects in our possession are in 18 kt gold.

Before continuing, a very quick hint that will allow us to understand the rest of the speech.

What is 18 karati gold? 

Pure gold is 24 kt, where 999/000 thousandths of a part are in gold. 18 karati gold means that 750/000 are in pure gold, while the remaining 25% is made up of other metals. The 18 karati is therefore an ALLOY.

It binds pure gold to other metals to make it more robust and easier to work with.

Having said that, why does it oxidize?

24 kt gold CANNOT oxidize, it is 18 kt gold that oxidizes , because among the metals used to make the alloy there is COPPER  which oxidizes on contact with air only.

But don't worry, to remove the oxide from your objects, just a polishing cloth and they will immediately return as new.