If there were twelve tribes then who were the Gentiles?

A Gentile is someone who is not a Jew, that is a descendant of any of the twelve tribes of Israel. If someone's parents have no trace of jewish ancestry at all, then that person is definitely a Gentile.

However, in Judaic law a person's jewishness is automatically passed on through the maternal line that is, through the mothers: "once a jew, always a jew".

This means that even if the mothers have converted from Judaism to another faith, they are still regarded under the jewish "Law of Return" as legally being jews, but if the fathers have married non-jewesses and have converted from Judaism to another faith, then his children are all Gentile (unless one of his sons or his sons' descendants then marries a jewess, in which case all theirchildren are also regarded as being jews).