The spin editor has a default minimum and maximum (0,100)

Workaround:
private void GanttViewElement_EditorInitialized(object sender, GanttViewItemEditorInitializedEventArgs e)
{
    var editor = e.Editor as BaseSpinEditor;
    if (editor != null)
    {
        editor.MaxValue = int.MaxValue;
        var value = e.Item[radGanttView1.CurrentColumn];
        editor.Value = value;
    }
}