Understanding the properties of young open clusters, such as the Initial Mass Function (IMF), star formation history and dynamic evolution, is crucial to obtain reliable theoretical predictions of the mechanisms involved in the star formation process. We want to obtain a list, as complete as possible, of confirmed members of the young open cluster Gamma Velorum, with the aim of deriving general cluster properties such as the IMF. We used all available spectroscopic membership indicators within the Gaia-ESO public archive together with literature photometry and X-ray data and, for each method, we derived the most complete list of candidate cluster members. Then, we considered photometry, gravity and radial velocities as necessary conditions to select a subsample of candidates whose membership was confirmed by using the lithium and H$alpha$ lines and X-rays as youth indicators. We found 242 confirmed and 4 possible cluster members for which we derived masses using very recent stellar evolutionary models. The cluster IMF in the mass range investigated in this study shows a slope of $alpha=2.6pm0.5$ for $0.5<M/M_odot <1.3$ and $alpha=1.1pm0.4$ for $0.16<M/M_odot <0.5$ and is consistent with a standard IMF. The similarity of the IMF of the young population around $gamma^2 $Vel to that in other star forming regions and the field suggests it may have formed through very similar processes.
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