Reference

Gouraud shading is the default method of lighting (mode 3) used by JK for level surfaces. Things are lit with gouraud shading only when lit with a dynamic light. Otherwise, they are solidly lit with a single value which appears to be averaged from nearby level lighting and sector lighting. More precise testing should be done to determine exactly how a thing is lit under regular level lighting.

A screenshot of a thing with lighting mode 3. Notice how it is not actually gouraud shaded.