I hiked half the GR20 in June — trip report — and can confirm it’s bloody tough.
Guidebook author Kev Reynolds calls the GR20 high-level route across Corsica one of the toughest 3 treks in Western Europe.
GR stands for Grande Randonnée, which means big excursion in French. It is a network of long-distance footpaths in Europe, mostly in France, Belgium, the Netherlands and Spain. GR20 is considered to be the most difficult of all the GR routes. Its Corsican name is Fra li monti (sometimes spelled wrong as Fra li monte), what means “across the mountains” in Corsican. …
The trail is well marked with red and white rectangles on rocks, boulders and trees. Feeder paths are marked with one colour. There are also small stone heaps along the path. It’s still easy to get on a feeder path or to lose the way, so having a good map is necessary.
photo from David Nobel’s trip report
Possible side trips
Monte Corona — ascent to the 2144 m high summit at the end of the 1st stage (or the beginning of the 2nd stage).
Monte Cinto — ascent to the highest summit of Corsica (2706 m) at the end of the 3rd stage.
Monte Rotondo (Monte Ritondu) — ascent to the 2nd highest summit of Corsica (2622 m) in the 7th stage.
Monte Renoso — ascent to the 2352 m high summit above the glacial lake of Bastiani (2089 m) in the 11th stage.
All those text grabs are from corsica.forhikers.com
The best section is in the north.

Kev Reynolds compares it in difficulty to two other challenging adventures in Western Europe:
• Pyranean Haute Route
• Tour of the Oisans (GR54)



