No era una chica a la que le gustara.
Look:
Era una chica a la que le
gustaba.
Era una chica a la que
no le
gustaba.
No era una chica a la que le
gustara.
Sometimes, in some sentences, subjunctive is used when you use the negative form. Actually I don't know the reason why this "no" at the beginning of the sentence provokes the change of the mode, but it's like this.
Te gusta el verbo... gustar, ¿verdad?
