What does 220 mean on a gold hallmark?

220 is not a gold hallmark.

The lowest number that is used as a gold hallmark is 333 (meaning 33.3% gold content, which is 8 carat, the lowest gold carat).

220 is actually the hallmark for bronze, which is a copper alloy called "Alloy 220" (or sometimes sold as "commercial bronze"), not gold.