They are different associations.

If the school identifies solely as "Gracie Jiu Jitsu" a.k.a "Gracie Humaita," it tends to be affiliated with Helio's line, primarily the Gracie Academy in Torrance, CA. The instructors may have trained under one of Helio's sons, probably Rorion or Royce if they're using the GJJ name, or under Rener or Ryron. There's also a chance that the school only has a loose affiliation, as the Gracie Academy does their whole seminar and certification thing, where you train a little bit with those guys and you're certified at a certain level.

Gracie Barra is an enormous worldwide association, and the name stems from the other of the two Gracie schools when they were still in Brazil. "Barra" (pronounced baha) refers to the area where the school was located. Gracie Humaita was affiliated with Helio (the Gracie Academy is an offshoot of that, and Gracie Humaita affiliation is currently run by Royler and Rolker Gracie). Gracie Barra was Carlos' school, and the affiliation is currently run by Carlos Gracie Jr. The original crop of instructors from GB would have trained in Brazil under the likes of Carlos Jr., Renzo Gracie, the Machados, and many many others, who have since left to form their own schools. Most of the GB instructors nowadays are probably another generation removed, and may have received their black belts from the crop of instructors I was just talking about.

In other words, what I'm trying to say is that they're two different affiliations. If they're saying "Gracie Jiu Jitsu" you should closely inspect the instructor's credentials, because of their affiliate system. Those schools probably won't focus much on MMA or sport grappling, and will claim more of a self defense focus, and an old school feel to their techniques. Gracie Barra has a fundamnetals class where they teach self defense stuff, but they are much more sport oriented, and I don't mean that to say that it's less effective. Probably more effective if you're pitting a student from one school against the other. In other words a sport oriented school is training to grapple against primarily other BJJ practitioners. Doesn't mean one is better than the other, it comes down to the instructor.