Where the Gros Michel was thick-skinned and sweet, being much firmer and having a stronger taste and smell, the Cavendish is larger and more mildly flavored.
The Gros Michel became a popular fruit worldwide by the mid-1800s but was almost wiped out by the fungal Panama Disease during the 1950s. Sellers then switched over to Cavendish.
You can find Gros Michel bananas, but they’re rare. Most grocery stores sell Cavendish bananas, which also began suffering from a new strain of Panama Disease in the 1990s.