CJB20 It seems that a direct replacement bus is now running from Geneva to Milan. You can book this bus on raileurope.com with the Eurail reduction, it costs 13 €.
You can also take a train to Domodossola and the replacement bus from Domodossola to Milan, but it seems not possible to book the bus with the Eurail reduction. You can book this bus on trenitalia.com, it costs also 13 €.
If you prefer make the entire trip by train, I suggest to take the route Geneva - Lucerne - Lugano - Milan (without reservation with a regional train between Lugano and Milan) or Geneva - Zurich - Milan (with mandatory reservation in the Eurocity trains Zurich - Milan).