You can always initialize the Guid in a constructor.
public partial class Event
{
public Event()
{
ID = Guid.NewGuid();
}
public System.Guid ID { get; set; }
public int AccountID { get; set; }
public string ExternalID { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public string Location { get; set; }
public Nullable<System.DateTime> StartTime { get; set; }
public Nullable<System.DateTime> EndTime { get; set; }
public Nullable<bool> AllDay { get; set; }
public Nullable<System.Guid> ImageData { get; set; }
public string ImageUrl { get; set; }
public virtual Account Account { get; set; }
public virtual FetchedFile FetchedFile { get; set; }
}