Why were the Gambinos not included in the series?

Tony talks about John Gotti and Gravano early on in the series. So they do exist but they are never included in the series.

Why is this? It makes no sense.

In the imaginary world of the Sopranos, did the Gambino family become defunct before the series started in 1999? Or did they just go by a different name in the series?

Maybe it was enough to allude to them without specifically naming them? Especially at the time? Just spit balling

In the world of the show either Mangano, Teresi, or Paglieri took over the Gambinos in the 90s. The other two families, Lucchese(Lupertazzi) and Bonanno (Joey Massino), are spoken for.

If they ever come out with another sopranos movie a sequel to the Saints of Newark that takes place in the 1980s, early 1990s, they could indulge in that storyline a little bit and explain how all that happened. Also they could easily introduce some very interesting new characters.