How does it make sense for someone to be severed after they die? If they only exist at Lumon, where is Lumon getting the body? Where would they put the chip? To me, it seems more likely that Gemma was targeted as a good subject for testing when she and Mark went to the fertility clinic. Perhaps the trauma surrounding her infertility, together with the Chikai Bardo cards as a kind of assessment, somehow suggested Gemma's suitability for severance testing. So then Lumon engineers the car accident, snatches her from the car and replaces her with a dead body too badly mangled for an open casket, conceals the truth with the help of Lumon-owned coroner and undertaker (, Lumon pretty much owns the town), and moves her onto the severed floor. Perhaps a Lumon employee (someone at the fertility clinic?) was around to talk Mark into working on the severed floor as a way of coping with her death. This would also serve to test whether two people who are close as outies would recognize each other's innies.