Hey osfox!

As said in this post, the "Gene hunter" ability is linked to the name of the faction's FactionAffinity, which is why it's currently impossible to have this gameplay feature in a faction trait.

However, the other factions' gameplay is based on the descriptors they give, so you probably do that with other faction gameplay.

You can look at the FactionTraits FactionTraits[Major].xml in the game's files to see how the "regular" traits are defined, 

and look at the FactionAffinities in FactionTrait[Affinity].xml to see which SimulationDescriptorReference you may need to include (stuff like "AffinityGameplayCravers").