As I said before, Unity uses Mono version that only implements .NET Framework 3.5. Support for .NET 4.6 in Unity 2017 is still experimental. So you can’t use it in production. In Unity projects you change target .Net framework in the Unity Editor, not in Visual Studio.
EyeX SDK won’t provide you more data than Tobii Unity SDK. Could you please explain why do you need it in Unity?