The reason you have to remove all gel nails and nail polish is because your fingernails are a good sign of whether you’re getting enough oxygen.
Quite simply, they turn purple if you’re not, or they’re a nice pink colour if you are 🙂
When you’re under general anaesthetic it’s a super good way for them to keep track of it, along with all the snazzy gadget ways of telling 🙂
Good luck!