Mixer design and optimization in ADS

I'm designing a mixer in ADS and I'm trying use its optimization function to help me.

I can set optimize for noise figure, ip3_out, but I'm not able to set a goal for conversion gain as ADS does not have a block that calculates this (AFAIK). So I tried making a equation for conversion gain but the optimization tool does not accept expressions as goals, I get an error. Any idea how to set goals for conversion gain?

SALUT

I hope that can help you?