Helsinki centre is hollow like a Swiss cheese. A great deal of the underground space is reserved for parking, but there are also many underground pedestrian connections and shopping malls. The Railway station, shopping malls Forum, Kamppi and City-Center and department store Stockmann are all connected to each other. This underground network extends all the way to Kluuvi and Kaisaniemi, but the most distant parts are for service traffic only. If you go under ground here, you can reach geocaches GCZG0F, GC23VEY, and some others too without popping up on the ground earlier than about 50 meters from the cache. But unfortunately pedestrians are not allowed to enter here.
Actually the exact direct distance to GCZG0F is 642 m and to GC23VEY is 167 m (the final coordinates). This should define the location precisely enough if you have found both, but for your convenience the direction to GC23VEY is 258 degrees. I am afraid you have to calculate it's location first.
The location is quite public, and it will be difficult to reach the cache unnoticed. Also the GPS coverage is quite poor at the site. I apologize your inconvenience... The coordinates have been taken from OpenStreetmap. NOTE! The cache is not down "there"! You do not need to do anything dangerous or stupid to get this cache. The cache box is crearly visible from the right direction and distance.
I suggest you visit the reference point of this cache too. It has an interesting connection to the underground world, but no entry!