You'll find a bunch of Sierra employees in the PD's computer system, such as Jon Bock, Jane Cardinal, Jeff Crowe, Mark Crowe, Carlos Escobar, Desi Hartman, Jane Jensen and more.
Look at the book store window. Jane Jensen (the woman who wrote and designed each Gabriel Knight game) can be seen. Also in the window is Gabriel's book he wrote about the Voodoo Murders. If you have Gabriel look at it he'll say "Wow, isn't he some famous author or something?"
When Gabriel calls to the Travel Agency, one of the options is Anderson, Indiana. Game designer Jane Jensen received a BA in Computer Science from Anderson University in Indiana. This Easter Egg also appeared in the original Gabriel Knight 1.