Hi anagabygh
The GA Travelcard is valid until Domodossola. So you only need a ticket from Domodossola on.
If you purchase a ticket for a EuroCity (EC) starting from Domodossola the reservation is also starting from Domodossola, so between wherever you’re starting until Domodossola you do not have a seat reservation. So if someone already sits there, you will have to wait until Domodossola.
If you take a regional train from Domodossola you won’t have to worry about a seat reservation.
I would suggest to buy the ticket from your starting location and add your GA Travelcard. You will only be charged for the Italian part, yet you get a seat reservation starting from where you take the EC
Hope this helps and best regards
Zebis