How can the people of Gog and Magog say, "If God wills," when they are disbelievers?

The stories mentioned about the exception are correct. The first is the story of the Prophet, peace be upon him, in Surat Al-Kahf. The second is the story of Solomon, peace be upon him, mentioned in the Sahihayn, where he forgot to say "Insha'Allah" (if Allah wills), which resulted in what he desired not coming to pass. As for the third, it is the story of Gog and Magog. It has been narrated in an authentic hadith (definition) that they will emerge when their leader says "Insha'Allah." The exception of disbelievers is not surprising, as many of them are monotheistic in their belief in rububiyyah (Lordship) and believe that everything is by the will of Allah, even if they have disbelief in other aspects.