The whole adoption thing gets glossed over a lot, but if you piece it together from the manga and fanbook info, it's pretty wild. Satoru Gojo wasn't exactly looking to be a dad. He was a first-year student himself, maybe eighteen, when he found out about this Zen'in clan kid, Megumi, being basically sold off by his awful father. Gojo saw the kid's potential and the trash situation he was in, and his solution was… to buy him. Or, more accurately, he went to Toji Fushiguro and said, 'Hey, I'll give you a ton of money for the rights to the kid,' knowing Toji would take the cash and bail.
It wasn't a warm, fuzzy foster care placement. Gojo secured Megumi's safety and future, then dumped him with his sister Tsumiki and their mom. He paid the bills, checked in, but mostly left them to have a normal-ish life. He changed Megumi's life by giving him an out from the Zen'in clan's toxic destiny. Without Gojo, Megumi would've been raised as a tool for that cursed family's politics. Instead, he got to grow up with Tsumiki, which is why he's so fiercely protective of her. The real change wasn't Gojo teaching him jujutsu—that came later—it was Gojo giving him a choice. He essentially told Megumi, 'You can be a Zen'in weapon, or you can be whatever you want. I'll make sure you have the power to pick.' That's a massive shift for a kid in the jujutsu world.