How is it fair that most people throughout history haven’t had the gospel? And how will they be judged?

This life is a test of our obedience (see Abraham 3:24–26), but most people haven’t necessarily had all of God’s commandments. So, what are they being tested on? “God judges men according to the use they make of the light which He gives them” (Teachings of Presidents of the Church: Joseph Smith [2007], 405).

Those without the gospel still have the Light of Christ (or a conscience), which tells us right and wrong. They will be judged by how they used this Light and any truths they received. “Those who have lived without law, will be judged without law” (Teachings: Joseph Smith, 404), and after hearing the gospel in the spirit world, those “who would have received [the gospel] … shall be heirs of the celestial kingdom” (D&C 137:7).

And on the question of fairness, remember that at the Council in Heaven “we were all present and saw … the plan of salvation made, and we sanctioned it” (Teachings: Joseph Smith, 209). We knew what we were agreeing to, regardless of where and when and in what circumstances we would be born. “We shall all of us eventually have to confess that the Judge of all the earth has done right” (Teachings: Joseph Smith, 404).