Interesting biology question.
In order to answer this question, I hope you understand what is mitosis: cell division that creates two diploid "daughter" cells that are identical to each other. Gametes, remember, are the reproductive cells (egg cells and sperm).
If human gametes were made by mitosis, which is not true, because gametes are made in meiosis, they would simply be diploid with somatic cells. The two gametes would have 92 chromosomes, as each cell has 46 chromosomes. And after each generation, the number of chromosomes would keep doubling, which would be very weird. Remember that the normal amount of chromosomes that a human being has, is 46. Maybe humans would have mutations if that happened.
