You can currently use Tobii.StreamEngine (low level api). We are also soon releasing an update for our high level sdk and unity sdk that will both have head tracking support.
https://www.nuget.org/packages/Tobii.StreamEngine.Native/
https://www.nuget.org/packages/Tobii.StreamEngine/
This is a low level API so please only use it if you know why you are doing it. Very little support is going to be provided.