My wife and I stayed for three nights at Gustavo's place. Gustavo was very welcoming and accommodating, both agreeing to meet us an hour earlier than advertised and offering to let us stay an hour later. He met us at the apartment, showed us around and spent a lot of time making suggestions for local restaurants and bars.
The apartment itself is ideally located for a first-time visitor: right in the old town, but on a quiet street on the top floor. We were able to walk everywhere without a problem. The apartment is very clean, with a nice bathroom, terrace and bedroom. There is also plenty of space, perfect for a couple.
Thank you again Gustavo, if we return to San Sebastian we know where we are staying!