Which is harder 22 karat gold or mild steel?

22 Karat gold is not pure gold (91% Gold) so it depends what the other 9% is.

For instance:

Jewellery gold is 91% Gold 5% silver 2% copper 1.33% Zinc

The Young's modulus of Gold is 79 GPa mild steel quotes a maximum of 210 GPa

(Bigger is stronger)

Therefore Gold at 22 karat is not stronger than mild steel.