Whilst working on my gigameter, it's got me thinking about it's use in the films and in the GB universe. The equipment information page states that it reads psychomagnotheric energy expressed as a GEV readout. I've read that people believe it's purpose is to find mood slime but I think it's a little more than that. I suggest that the gigameter is used to measure whether a ghost is docile or not, the psychomagnotheric reading helps the GB better understand what they're dealing with and act accordingly. For example, Slimer was not an aggressive nor particularly evil Ghost, he's friendly and generally docile only attacking when attacked himself. His GEV reading would be low. Gozer and Vigo, who's soul purpose is world domination and destruction, with very evil tendencies would give a high reading. If we're taking the IDW comics into account, collected volume 5, The New Ghostbusters, when the team is battling Dibello, Kylie uses the gigameter to source where he is. Certainly this isn't about just mood slime. Realising his evil nature as a serial killer, they were able to use the information to understand Dibello's motives and defeat him.
Looking at GB2, there is a running theme throughout the course of film of people being nasty to one another, from the opening scene with the guy arguing about his parking ticket etc, discovery of the evil mood slime, "6 million completely miserable assholes" etc. The GB would be measuring this, clearly they had been to create the positively charged mood slime as a counter measure. It makes sense that the gigameter would be used to monitor this.
Just a thought
You did not use...the magic word....