Walk to the Helsinki city center Kamppi bus terminal (ask hotel official how to find) and take bus number 102 or 103 and leave the bus at the Otakallio (Otakallio stop) which is the fourth stop after Nokia headquarters (including the Nokia stop). Walk from the stop to GTK headquarters.
Open map (pdf)
Note! Single tickets are available from ticket machines, bus and tram drivers as well as conductors on commuter trains. Day tickets for 1 to 7 days are available from ticket machines.
The local transportation (HSL) main pages: http://www.hsl.fi/EN/Pages/default.aspx
The bus timetables: http://aikataulut.ytv.fi/linjat/en/seutu.html Select Regional Lines, 102 or 103
Information on ticket prices: http://www.hsl.fi/EN/ticketsandfares/singleticketsanddaytickets/Pages/default.aspx
GTK, Espoo, Otaniemi map