I haven’t done any research, so I’ll just leave a comment. Based off my knowledge, You can attempt to catch Red Gyarados as soon as you have a Pokémon with Surf and the Ecruteak badge (the 4th gym). The way you would transfer to gen VII is by using the Pokémon Transporter app in conjunction with Pokémon Bank, both available for download from the Nintendo estore. And finally, I believe Gen II mechanics with nature work like Gen I. You have to give your Pokémon a certain amount of experience to get the nature you want.