Is what was narrated about Gabriel (peace be upon him) – when he was permitted to measure the width of Paradise, and he flew for 900,000 years, then stopped and saw a houri in her palace, and she said to him: "O Gabriel, do not tire yourself; you have been flying within the bounds of my kingdom since your first takeoff" – a hadith or an athar?

The statement mentioned in the question, regarding Paradise being as wide as the seven heavens and the seven earths, in addition to the heavens and earths created for angels and jinn, has no basis in the noble Prophetic Sunnah. Rather, it is among the statements circulated by some Rafidah books and websites. The Holy Quran describes Paradise as being as wide as the heavens and the earth, as in His (Allah's) saying: "And hasten to forgiveness from your Lord and a Garden whose width is the heavens and the earth, prepared for the righteous" (Aal 'Imran/133), and His saying: "Race toward forgiveness from your Lord and a Garden whose width is like the width of the heaven and earth, prepared for those who believed in Allah and His messengers." (Al-Hadid/21). These verses indicate the immense vastness of Paradise, and that the description of its "width" signifies an exaggeration of its vastness, and that only Allah knows its length. Furthermore, the lowest in rank among the people of Paradise will have the equivalent of the world and ten times its like, which also indicates the immense vastness of Paradise. It is not permissible to lie about the Prophet (peace be upon him), and Muslim scholars have agreed on the prohibition of narrating fabricated hadiths.