  Runaway stars are ejected from their formation sites well within molecular
cores in giant dark clouds. Eventually, these stars can travel through the
molecular clouds, which are highly inhomogeneous. The powerful winds of massive
runaway stars interact with the medium forming bowshocks. Recent observations
and theoretical modelling suggest that these bowshocks emit non-thermal
radiation. As the massive stars move through the inhomogeneous ambient gas the
physical properties of the bowshocks are modified, producing changes in the
non-thermal emission. We aim to compute the non-thermal radiation produced in
the bowshocks of runaway massive stars when travelling through a molecular
cloud. We calculate the non-thermal emission and absorption for two types of
massive runaway stars, an O9I and an O4I, as they move through a density
gradient. We present the spectral energy distributions for the runaway stars
modelled. Additionally, we obtain light curves at different energy ranges. We
find significant variations in the emission over timescales of $\sim$ 1 yr. We
conclude that bowshocks of massive runaway stars, under some assumptions, might
be variable gamma-ray sources, with variability timescales that depend on the
medium density profile. These objects might constitute a population of galactic
gamma-ray sources turning on and off within years.
