What is the triangular gable on a neoclassical building called?

The triangular gable on a neoclassical building is called a pediment.

A pediment is the classical, usually triangular upper section of a building’s façade. It sits above a horizontal entablature or cornice and often crowns a portico supported by columns. The recessed triangular field inside it is the tympanum, which may contain sculpture, reliefs, inscriptions, or a clock.

Pediments originated in ancient Greek temples, where they formed the front and rear ends of pitched roofs. The Temple of Artemis at Corfu, built around 580 BC, is among the earliest known examples. Roman architects adapted the form, and Renaissance, Baroque, Neoclassical, and Beaux-Arts designers revived it centuries later.

A common mix-up is calling the entire entrance porch a pediment. The porch is a portico; the pediment is the gabled element above it. Pediments are not always triangular: segmental, broken, and open versions also appear in later architecture.