What's the difference between a Gran Fondo and an Étape?
All three terms refer to mass participation events that are open to amateur cyclists. They are timed events but they are not races, although some of them are very competitive and there are sometimes prizes on offer for the best times.

The Etape du Tour is French. Each year two stages from the Tour de France are replicated on closed roads for amateur cyclists to enter.

Gran Fondos are Italian. They are timed events with one of the best known being the Maratona Dles Dolomites.

Century Rides is the term used in the United States. 

In the UK and in France they are also called Sportives or Cyclo Sportives.

These events are very popular with cyclists and the largest attract thousands of participants.