Located in oporto centre you can easily walk to every monument (five minutes to city hall!). Undoubtedly one of the best places to experience the city and the best if you go out at night.
If you want to go to the airport or surronding area there are two subways (metro) stops nearby.
From airport to GZ hostel by public transportation: Take the metro at airport (line E toward Estádio do Dragão) and get off at Trindade metro stop (20 minutes). Then walk 500 meters to GZ.
From airport to GZ by car: Take the following main roads (A28 to Boavista and then follow Rua Damião de Góis). The GZ its located in Rua Antero Quental, 11 on your right (16km, 15 minutes).
Check the map to see the location of the GZ hostel probably one of the best oporto locations.