When I run Servants of Gaius I often use the Horror Show monster creation rules in Chapter Seven to create new threats for my players. Sometimes I just use existing monsters from the Horror Show monster chapter as well. An example of this is when I brought werewolves into my campaign in Germania.
When we designed Horror Show, we had a sense that the monster section might be useful for our other games. At one point I was planning a Crime Network campaign that turned into a horror campaign midway through (but I didn't have the heart to make that kind of switch). Mainly I found the rules useful in Servants of Gaius. So we decided to bundle them as a PDF package.
Now you can get PDFs of Servants of Gaius, Horror Show and the Guide to Aegyptus together for $6.50 at places like RPGNow.
You can find the Servants of Gaius bundle here: SERVANTS OF GAIUS BUNDLE
I think this is a very good combination. Servants of Gaius and Horror Show blend quite nicely. The key is to use chapters Seven and Eight from Horror Show so your party can face werewolves, vampires and monsters of your own creation.