Hello and thank you for using GraphStream.
To compute the union of two (ore more) graphs you can use the methods merge or mergeIn defined in org.graphstream.graph.implementations.Graphs class
There is no direct method to compute an induced graph as far as I know but it is not difficult to code one. Maybe the simplest way is to clone your graph and to remove the nodes that do not belong to the set.
Hope this helps,
--
Stefan