Commentary
As for the pronoun of the word: بَعْضَهُمْ (ba` dahum) in بَعْضَهُمْ يَوْمَئِذٍ يَمُوجُ فِي بَعْضٍ (And on that day, We will leave them [ Ya’ juj and Ma'juj ] surging over one another — Al-Kahf 18:99 ⧉), it is obvious that it reverts back to Ya'juj Ma juj. Then, as for their condition described therein - 'surging over one another' - it is also obvious that this will be their state when the passage lies open for them and they will be hastening their way from the hilly heights down to the ground. Commentators have suggested other probabilities as well.
The pronoun in: فَجَمَعْنَاهُمْ (fajama'nahum: Then We shall gather them together - 99) is reverting back to the creation at large, human beings and the Jinn. The sense is that the entire creation of human beings and the Jinn who have been obligated with the percepts of the Shari` ah (mukallaf) shall be gathered together on the plains of Resurrection (al-Hashr).