Regarding the gang rape- that's hard. Could you start with 'sometimes babies are made because the mum and dad plan their babies, sometimes the mum and dad don't plan the baby, but a baby gets made, sometimes babies are even made in a test-tube! There are all sorts of ways babies get made. Some babies are made when men aren't very kind to a mum, and that can make it very difficult for that mum to be a good parent. It's not up to a baby how they get made, and it's never a babies fault how they get made. When you were made, some men were not kind to your mum, and them being unkind will have made her sad. You didn't make her sad. She grew you and -insert anything positive here-and brought you into the world." I don't know if that could be a start? It means that it's never 'big news' and as they get older, and understand sex/rape, which probably isn't until teens, you can clarify, but it's not been glossed over that their genetic dad wasn't nice to their mum. I think it helps to start away from the narrative of 'babies are made when a mummy and a daddy love each other very much...' because that's not true. The books I've made have the biology in there, that a man makes a sperm, and a woman makes an egg, and that's how babies are made, then they grow for 9 months in a woman's tummy/womb, you grew in BMs womb in her tummy, but sadly she couldn't look after you because of xyz... I even drew sperms and eggs! I know many people are not happy with the biology too young, but I felt it was hard to explain what 'biological parents' means, with no biology!
I'd also advise being really honest with yourself about how you feel about your child's history, which I'm sure you've already been through. But if you feel any shame about the fact you have a child conceived possibly by gang rape, then they will pick up on that shame. Having counselling to identify your own feelings, and reactions, could be helpful, so you can be a neutral when it comes out.