Is the Gulf of Mexico Part of the Atlantic or Caribbean? Neither—It's a Distinct Body

Neither. The Gulf of Mexico is a separate oceanic basin bordered by:

• United States (north, east)
• Mexico (south, west)
• Cuba (southeast)

It connects to the Atlantic via the Straits of Florida and to the Caribbean via the Yucatán Channel, but it is a distinct marginal sea with its own circulation, ecology, and geology.