Named after their prototype, γ Cas stars are a subset of Oe/Be stars emitting hard, bright and variable X-rays. The origin of these peculiar X-ray emissions is still in debate. Proposed scenarios include accretion onto a compact object, magnetic interaction of the Be star with its circumstellar disk, and collision between the wind of a stripped helium star companion and the Be disk. In this work, we model fluorescent Fe-K line emission from the rotating circumstellar disk and the photosphere of the Be star. The fluorescent Fe-K line is very sensitive to the physical conditions near the X-ray source. Thus, this line will help to better understand the geometry of the ionizing source and the X-ray emission mechanism in γ Cas stars. Athena/X-IFU will unveil the morphology of these lines, thereby offering robust diagnostics to distinguish between the various scenarios.