General:
* Plump
* Robust
* Healthy
* Well-nourished
* Thriving
* Strong
* Vigorous
* Full-bodied
* Full-figured
More specific:
* Rubenesque (referring to a full, rounded figure)
* Portly (referring to a stout or corpulent person)
* Curvaceous (referring to a woman with a full, rounded figure)
* Voluptuous (referring to a full, sensual figure)
* Opulent (referring to a person who is wealthy and lives in luxury)
The best antonym will depend on the specific context in which "gaunt" is used.