Johnny Cade is the smallest member of the greaser gang and, next to Ponyboy, the youngest. He has the worst home life of them all: His parents argue all the time, and his father beats him. He has previously been attacked and badly beaten by a gang of Socs, so he now rarely travels alone and has taken to carrying a knife. He is sensitive, like Ponyboy, and somewhat withdrawn. He is akin to a team mascot--the favorite of all the boys, especially Dally, who has few close friends. Although part of the boys' concern for Johnny comes from a sympathetic nature, Johnny is the unifying force between many of them. The gang needs Johnny as much as Johnny needs them.
Johnny is like a rope pulling the whole gang together. They feel as if it is there duty to protect him, therefore they avoid a lot of bickering by protecting him. He is badly beaten at home and the gang feels as if Johnny is thier little brother. Everyone can talk to him and he is a good friend. With Johnny gone, the gang would grieve as they did near the end of the book. If any one else died, as Dally did, the gang would be sad but there would not be much difference to thier gang. Thats what Ponyboy means.
"We could get along without anyone but Johnny"
When Ponyboy says this he expresses how much everyone values Johnny being around and how much they need him to tie the gang together, because after all Johnny is the gang's pet. Everyone in the gang loves Johnny, and when he dies the gang begins to fall apart. That is when Dally dies because Johnny was the only thing Dally loved and he had no reason to live.
Johnny Cade brought the Greasers together and made them a social group and he's the group pet. So Johnny dieing is a big impact on the group.