Pony comments that his gang members are like family. This is because they are all very close since they have to depend on each other for everything. Pony goes to the movie by himself, and feels nervous because the Socs often jump greasers who walk alone.
Or I could have gotten one of the gang to come along, one of the four boys Darry and Soda and I have grown up with and consider family. We're almost as close as brothers; when you grow up in a tight-knit neighborhood like ours you get to know each other real well. (Ch. 1)
The gang is not just a loose group, and not just a bunch of trouble makers. They are exceptionally supportive of one another, and will come to the aid of any gang member. Pony and the other greasers often do not go anywhere alone.
Pony also explains that his own family is different because his parents died in a car accident, leaving his brother Darry to take care of his two younger brothers.
Sodapop... a dropout so he could get a job and keep me in school, and Darry, getting old before his time trying to run a family and hold on to two jobs and never having any fun... (Ch. 3)
Darry had to become a father to his brothers, and Soda dropped out of school. Pony felt the weight of Darry’s expectations. It often seemed like Darry was too strict and overbearing. He wanted so badly to keep Soda and Pony out of trouble that he went overboard with Pony. The death of Pony's parents also means that he and his brothers rely more on the gang.
The significance of the greaser gang being family is obvious when Johnny is in the hospital dying. Johnny’s doctor wants to limit his visitors.
The nurses wouldn't tell us anything about Johnny and Dally, so Darry got hold of the doctor. The doctor told us that he would talk only to the family, but Darry finally got it through the guy's head that we were about as much family as Dally and Johnny had. (Ch. 7)
The doctor realized that the greasers were Pony’s family and allowed them to visit with him anyway. After all, he was dying. He didn’t even want to see his own mother. The gang meant more to him than his real family, which was abusive and distant.
Families come in all shapes and sizes. It takes more than a mother, father, and kids to be a family. Family is people who care about you and look out for you. Pony's gang of greasers served that purpose.