OnGcStart.cs

private void OnGcStart(EventWrittenEventArgs e)
{
    // This event is received after a collection is started
    var newGC = BuildGCDetails(e);

    // If a BCG is already started, FGC (0/1) are possible and will finish before the BGC
    //
    if (
        (GetFieldValue<uint>(e, "Depth") == 2) && 
        ((GCType)GetFieldValue<uint>(e, "Type") == GCType.BackgroundGC)
        )
    {
        _gcInfo.CurrentBGC = newGC;
    }
    else
    {
        _gcInfo.GCInProgress = newGC;
    }

    // forthcoming expected events for gen 0/1 collections are GCGlobalHeapHistory then GCHeapStats
}

private GCDetails BuildGCDetails(EventWrittenEventArgs e)
{
    return new GCDetails()
    {
        Number = (int)GetFieldValue<uint>(e, "Count"),
        Generation = (int)GetFieldValue<uint>(e, "Depth"),
        Type = (GCType)GetFieldValue<uint>(e, "Type"),
        Reason = (GCReason)GetFieldValue<uint>(e, "Reason")
    };
}