UnityEngine.GameObject not marked as Serializable?

Are you trying to use your own serialization interface? Google your question, there are several answers, threads, and other topics dealing with problems when trying to do that.

Other than that, my wild guess would be, don't use [SerializeField] or other related attributes above objects that are already serialized by Unity, include a "using UnityEngine;" instead of accessing it via UnityEngine.GameObject, etc.