Gustavo's apartment is situated right in the Old Town, half a block from the river. It is on a narrow street with lots of little shops and bars - fun to do our grocery shopping with the locals! It is within walking distance of the bus station, where the bus arrives directly from Bilbao airport (75 minutes). We walked to all the local pintxos bars, and the seafood restaurants on the quay.
The apartment itself is fairly small, but designed to make maximum use of the space. The terrace is probably the best feature, it almost doubles the living space, and we ate breakfast and lunch there most days. There were two of us, and we didn't feel on top of each other at all. There is a dishwasher, and a bonus, a washing machine.
Gustavo is charming; he met us at the apartment, explained everything clearly and supplied milk, coffee, fruit and a couple of beers to start us off. He was very prompt to answer any questions we had, either by phone or email.
Gustavo speaks English well, which turned out not to be the norm, unusual in Europe, so be prepared to use your high school Spanish.
We loved San Sebastián, and if ever we return we would definitely stay with Gustavo.