The carved stone ornament at the top of a Gothic pinnacle is called a finial. It forms the pinnacle’s crowning termination, emphasizing the upward thrust that is central to Gothic design.
A pinnacle is a small, often spire-like architectural feature placed on a buttress, tower, parapet, or gable. Gothic builders used pinnacles both decoratively and structurally: their weight helped add compression above flying-buttress systems, improving the balance of the church’s walls and vaults.
The finial could take several forms, including a carved leaf, flower, cross, or other pointed ornament. It should not be confused with a crocket, which is a repeated leaf- or bud-like decoration attached along the sloping edges of a pinnacle or spire. A boss, by contrast, is generally a projecting ornament at the intersection of ribs or mouldings.
Terminology can vary slightly between architectural traditions, and “finial” is also used for the top ornament of furniture, roofs, poles, and other objects. In the Gothic architectural context of this question, however, it specifically identifies the pinnacle’s terminal ornament.