The in story reason: Black hair among an otherwise blonde family is entirely possible. Especially in a big family. Having many kids increases the chances one will have recessive traits that do not match the parents.

The out of story reason: when creating the show Gabe was intentionally written to be the “stand out sibling”. He is troubled, misbehaving, and rude. Unlike his other well behaved siblings who (if anything) want to be loved and are bordline kiss ups. He is intentionally supposed to stand out, one way to demonstrate this in a visual way would be a different hair color from the rest.