I'm trying to find logic, even though some players in tuz mafia thought me not to.
We had a situation when one of Zach or Janna was going to be lynched. Since we know that Janna was a G, we know that the Gs had a problem. They didn't stay passive.
Now, did they try to save Janna by voting for Zach, or was Zach also a G and the Gs tried to lynch you? The third option is that they spread their votes among Janna and Zach (but this also goes in the "Zach is a G" category).
Now it's up to you to make a conclusion. If you think that Zach is a C, then you have no option but to look for the Gs among those who voted for him.