Is the story true about the man who gave his food to a strange woman, whereupon Jibreel (peace be upon him) gave him glad tidings of Paradise, and does Jibreel (peace be upon him) descend upon all human beings?

We have not found the story mentioned, and it does not establish a legal ruling. What suffices for it has been established in the Sunnah, such as what Al-Bukhari narrated from the Prophet, peace and blessings be upon him, concerning stories of sins being forgiven due to giving water to a thirsty dog. As for Gabriel, peace be upon him, descending upon anyone after the Prophet, peace and blessings be upon him, for purposes other than revelation, this is an unseen matter that cannot be proven, and the default is its non-occurrence.