Done the GR20 twice. Personally, I think it is a bit overrated, partially because of its popularity (mind you, it still is a great - and very tough - walk). Europe offers better walks.
In August, expect:
- Hot (check this site - note that Calvi is pretty close to the trailhead (coastal): http://www.tutiempo.net/en/Climate/Calvi/77540.htm ). Keep in mind that Calvi is at sea level, whilst the GR20 crosses a mountainous region.
- Sometimes violent thunderstorms in the afternoon
- A lot of hikers! Do not except solitude on the GR20, especially in August (on the other side: nice to meet other people from around the EU, including English speakers).
I always take an umbrella but this will be rendered useless during a thunderstorm, so I take a lightweight poncho as backup.
No need to take a stove & fuel on the GR20. ALL camping’s have an – outside – cooking system at no extra charge (in 2011, camping was € 7 pp. Wild camping is prohibited). Infrastructure is pretty good: virtually all camping’s have a refuge and a small shop. It is also possible to get breakfast/lunch/diner (don’t expect a classy restaurant meal though).
Water is often not a problem but take a good map/guidebook with you as some water sources are hidden a way. Due to the heat, I would set off with 1,5l and fill up at each occasion. I only had one day whereby I should have taken 3 liters or so: when I doubled the leg from Coruzza – Tighjettu, using the old – poorly indicated - route (instead of descending to Asco).
Leave the mosquito net and repellent home. Gloves, balaclava, wind pants and other warm clothing are also not necessary, except a microfleece during the evening.
I would take extra batteries for you headlight as the heat might tempt you to leave early, at sunrise (thus implying to pack up while it is still dark).
Apart from your kit:
I would advice to walk north -> south as this allows to climb most –morning - stretches in the shade.
When travelling light, one can easily double lots of stretches. Last time, I did the GR20 in 8,5 days. Keep in mind that some stretches involve some degree of rock scrambling.
Last but not least: the route between ref. d'Usciolu and ref. d'Asinao has been deviated Matalza for commercial reasons. The typical red-white way marks have been painted over with grey on the ancient route. The old route is way better (a.o. it crosses the top of Monte Incudine) and will save you a day. The trick is to recognize the spot where the old route branches away from the old. Afterwards, just follow the grey way marks. Remember: after Usciolu, the old route branches away when reaching a T-crossing (old route to the left, new route – with sign mark to Zicavo – goes to the right). It will be the ‘talk of the refuge’ at Usciolu & Asinao, so you could ask around before setting off).
Good website: http://corsica.forhikers.com/gr20