Over the years I have done the GR20 six times, N-S, S-N, climbing peaks, sleeping on peaks, slight variations each year. Last year I did the JMT. Each time I went in June (not JMT): main advantage is the snow is still melting and so all the springs marked on the Didier et Richard 1:50,000 maps could be found on the ground, albeit sometimes only a 6" pipe poking out of the rock. This meant I didnt usually carry any water, just hiked quickly from source to source. Also it was cooler, daytime 15-20'C, with night min of 7'C.
Rainwear : Having seen French men and women wearing ponchos nearly taking-off on high ridges , I prefer a light rain jacket and trousers, I guess you call them pants. 7 oz jacket and 3oz golight reed type trous will do. In the 6 traverses I only got rained on twice, briefly.
My times were 7days GR20, 13 days JMT. You might want to take longer, but these times dictated my fuel and food strategy. I took ALL my food from the UK, which meant it was all pre-packaged and ready to go. I flew into Calvi, Bastia or Ajaccio, hired a car, and drove up to the Col de Vizavona and cached 4.5 days if going north, or 2.5 days going south, plus a little for whatever peaks were planned that year. Then drove to the start and began walking, the same day the flight got in. Hertz have offices at all the airports, and Porto Vecchio. The hotel at Tarco accepted drop offs, and provided a taxi up to Conca. Taxi Calvi to Calenzana.
Regarding food, even if you don't take everything, I would at least take your own dehydrated stuff, tea, coffee, favourite energy bars etc. I drank from the springs entirely, did not purify anything, and did not get ill.
The route is well marked, over-marked in places, such that a red/white flash can be seen either in front or behind nearly all the time. To keep my route as "pure" as possible, I stayed on the high ground, avoiding Vizavona village, Haut Asco (use the old route) and the hotel at Col de Vergio. The route is much rougher than the JMT over long stretches, so that your average speed is likely to be 20% slower. It is a remote route, at least for Europe, and as I said, you can avoid all habitation if you want to, apart from half a dozen houses at Bavella.
In all my trips , I have never stayed in a hut or hut official campsite, instead bivying up high, (a tent is not good for stealth), starting at sunrise and finishing late, but with long brew stops in between. Beware the green of the hillsides is often not grass, it is more likely low thorn scrub, making pitching a tent impossible . Also beware that they may have tightened up the no free-camping rule, with "rangers" checking up.
No particular trail worries : hikers have been killed in forest fires, so no open fires; no bugs or wild animals, apart from a few hogs; but watch out for that thorn matting, I have spent quite a lot of time digging thorns out of my hands!