Yajuj and Majuj are also mentioned in Surah al-Anbiya. Quran 21:96-97 says that when they are let loose they will descend from every elevation, and the true promise will draw near.
An authentic hadith speaks about an opening in the barrier. Zaynab bint Jahsh, may Allah be pleased with her, reported that the Prophet ﷺ came to her alarmed and said that there is no god but Allah, and woe to the Arabs from an evil that has come near. An opening had been made that day in the barrier of Yajuj and Majuj like this, and he made a circle with his thumb and the next finger. She asked whether they could be destroyed while righteous people were among them, and he said yes, when evil and corruption increase (Sahih al-Bukhari 3346 and 7135, Sahih Muslim 2880).
A similar report from Abu Hurayrah, may Allah be pleased with him, mentions the same opening (Sahih al-Bukhari 3347, Sahih Muslim 2881). The hadith is a warning to keep away from corruption. It gives no dates or details, and we should not add any.