Gamma-ray binaries, which contain a massive star and a compact object, are very interesting astrophysical laboratories because particle acceleration and radiation/absorption mechanisms are modulated by the orbital phase. However, only 8 of such sources are currently known: 4 composed of an O-type star and 4 of a Be-type star, being the systems with an O-type star runaways with respect to their environment. Gaia DR2 provides us with useful information of positions, proper motions and distances for 1332 million sources. To search for new gamma-ray binaries we have studied a sample of 370 O-type stars from the GOSC catalog. In Gaia DR2 all sources are treated as single stars. Therefore, we could expect bad astrometric solutions for some gamma-ray binaries because Gaia DR2 does not fit the 7 orbital parameters for binaries. We have used the RUWE quality indicator provided by Gaia DR2 to distinguish between bad astrometric solutions (RUWE > 1.15) and good astrometric solutions (RUWE < 1.15). Among the 370 O-type stars we have found 36 with bad astrometric solutions, some of which could be gamma-ray binaries. We have also computed the peculiar velocities of the remaining 334 O-type stars and found 74 runaways, some of which could also be part of new gamma-ray binaries. We conclude with a short discussion and an outlook of future work.