In The Outsiders, what do Ponyboy and Two-Bit mean when they say they can get along without anyone but Johnny?

The sentiment that the gang cannot get along without Johnny is expressed over and over again in the story. And the boys all acknowledge that it isn't because Johnny is dependable in a fight or a good kid. They never quite articulate why it is but there are several clues as to why he is so important.

Everyone in the gang knows that Johnny has it really bad. Even Dally, who basically grew up on the street and then in jail, suggests that Johnny has it particularly bad. Not only do his parents not care about him, he was also beaten very badly and threatened in awful ways by the Socs when he was pretty young.

So the gang is the only real family he has and it gives them a real purpose for trying to be together and be a cohesive group. They want to be there for him and to protect him and it ennobles their affection for each other and their willingness to stand up to outsiders together.

This emotion is perhaps most deeply felt by Dally and demonstrated when he commits suicide by cop after Johnny dies in the hospital. For Dally, a really integral member of the greasers, Johnny was his most important reason for living.