I stayed here with a friend for the Oslo marathon. The living area in the apartment is clean, bright, and cosy. The bathroom is modern. There's a little balcony off the back of the flat looking into the quiet courtyard, and the living room looks on a wooded area, with the rest of Oslo in the distance.
The area of Sagene is 10 minutes walk from Gruner. There's a little square 2 minutes from the flat with a church in it, surrounded on the edges by a few little cafes and food shops - nice for a relaxed afternoon. 15 minutes by bus at the end of the road into town proper.
I would recommend for a relaxed stay in a quiet and friendly area of Oslo.