En este contexto, "le" refiere al sujeto, pero el sujeto no se escrito. En otras palabras, se puede escrito como: "A mi novia, ella no le gusta la pizza." Ella es el sujeto en este caso. "mi novia" en la frase "a mi novia" no puede ser el sujeto, porque "mi novia" es un objeto de una preposición; Se funciona en la misma manera como inglés.

Te sugiero que leas el siguiente link: https://modlang.unl.edu/docs/STC/gustar.pdf

Hi, David. It is weird, but actually the subject is "la pizza". "le" is an Indirect Object. In Spanish Indirect Objects can be identified as "me, te, se, le" pronouns most of the times and, unlike Direct objects, they refer to people. In this sentence, "le" and "a mi novia" are exactly the same. It could sound redundant, but actually this repetition is very common to emphasize the idea that she doesn't like pizza. This is called "dativo de interés". I hope this is not more confusing. Best,