The Grand Egyptian is scheduled to open in 2023. If the mummy parade that they did to accompany the opening of the Foustat museum a year or so ago is any indication, I imagine there will be a lot of fanfare when the GEM finally opens. Honestly, they have been saying "opening soon" for quite a few years. When we went to Egypt in 2020, they were claiming an expected opening in 2020; when we visited in 2021, they were saying 3rd quarter 2021; when I checked last summer, they were saying November 2022. Now, they're saying 2023. I'll believe it when I see it.
You will most likely see the Egyptian Museum in downtown Cairo. A huge amount has already been moved to the GEM, but there's still plenty to see, including the Tutankhamun death mask. Also, the museum itself, with its open windows and manually typed labels, is worth seeing. I really liked the old Egyptian Museum.
The mummies have all been moved to the new museum in Foustat -- the Museum of Egyptian History, or something like that.
Last I heard -- and it always changes -- is that they were planning to keep the downtown museum open even after the GEM opens.