Yes, you can, even without a Half Fare Card.
You can find all information on the Saver Day Pass, including terms & conditions of use, on this website: Carta giornaliera risparmio | FFS (sbb.ch)
There is also a map of validity (see section Download.) where you can see the validity of the Saver Day Pass, which equals the GA area. Arriving from Italy, the Saver Day Pass is valid from Chiasso or Domodossola.