i want to get node level using LayeredDigraphLayout like TreeLayout findTreelevel method .
You could override LayeredDigraphLayout | GoJS API to get that information whenever a layout happens. Iterate over the Layout.network.vertexes to get the LayoutVertex.node and LayeredDigraphVertex | GoJS API. Caution: some vertexes will not be associated with Nodes, so .node will be null, and the numbering of the .layer may seem reversed, with zero being the bottom.
Search the samples for an example usage.