The key to the gate can be found inside the courtyard, next to a skeleton and his toolbox.
The courtyard is a part of Salida del Sol north, the other part being Campanas del Sol. The courtyard is a square cross shaped area with a defunct fontain at the center. It has two exits to Salida del Sol south; the west one has a skeleton with entrance key near to it. The buildings in it are three stories high; the one in the northeast corner has a dining area in the basement.