The GR5 - Going Lightweight

The GR5/GR52 is a long hike - 600 km long, 32,000 meters up and the same down. You'll be carrying your gear all that way, so you obviously want to minimize it. You'll also be using your gear pretty intensely over 30+ days, so it's important it works well and you like it. You want to think long and hard about your gear list; this is the approach I used. 

We planned a lightweight trip for three main reasons. First and foremost is enjoyment - I enjoyed almost all the hiking on the trip, but I know that wouldn't have been the case if my backpack had weighed 10 kg more. Second, a lighter backpack means less pounding of the joints and hence a reduced injury risk. Lastly, an increased performance margin; if you need to deal with a crisis at the end of the day (e.g. having to walk to the next hut, getting lost, helping another party, horrible weather), you'll be in better shape if you've been carrying a light backpack.

The first step in going lightweight is staying and eating in huts, gites or hotels. This reduces your backpack to pretty much clothes, emergency gear and daytime food and drink. There are disadvantages in this approach (cost, busy huts, noisy dormitories, having to plan around the available accommodation) and it isn't for everyone, but it worked well for us. We also always took a picnic from a hut or (if we were in a valley) bought our lunch in the morning, so we only really carried food for that day.

The second step is is not to take stuff you don't need - and to be pretty ruthless about it. An obvious exception is safety equipment - here, you have to carry some stuff you hope not to use. I decided to take a whistle and decent bivy bag but against taking a gps and an emergency beacon (e.g. a spot locator) based on the margins I was comfortable with.  

My full backpack (without food and water) weighed 5.3 kg. Here is the gear I took, broken down into categories. All weights are in grams.

Carried Gear - 5285
  Backpack and Hydration System - 990
    Backpack (Simond Alpinism 22) - 730
    Hydration System (Source 2L) - 230
    Waterproof Clothes Bag (Sea And Summit) - 30
  Emergency Bivy Gear - 772
    SOL Escape Bag - 260
    Down Jacket (Patagonia) - 382
    Silk Sleeping Bag - 130
  Clothing - 1492
    Shell Layer - 440
      Waterproof top (Marmot Super Mica) 314
      Over-trousers (cheap, showerproof) 126
    Hut Clothing - 430
      Briefs - 60
      Socks - 60
      Merino Shirt - 137
      Trousers - 273
    Hiking Clothing - 622
      Light Top - 234
      Light Fleece - 212
      Socks - 60
      Gloves - 81
      Snood - 35
  Other Equipment - 2031
    Stationary - 754
      Topoguides (2) - 404
      Writing Materials, Puzzles - 180
      Passport, Wallet - 140
      Reading Glasses - 30
    Electronics - 318
      Phone + Charger - 180
      Camera - 138
    Toiletries - 357
      Towel - 127
      Toilet Bag (soap, toothbrush etc) - 110
      Sun Protection - 110
      Ear Plugs - 10
    Emergency Gear - 602
      Cliffbars (3) - 227
      Water Purifying Tables - 30
      Swiss Army Knife - 99
      Tissues - 25
      Compass - 34
      Whistle - 10
      Torch - 61
      Blister Cream (Friction Zone) - 80
      Advil - 20
      Waterproof Bag (Sea And Summit) 16

A few notes
1. There are 4 topoguides for the hike. I had two sent to the post-office in Modane (along with some minor things like extra sun protection) so I only needed to carry 2 at any one time (I sent the 2 'used' guides home when I collected the 2 'new' ones). This worked well.
2. My backpack was light, compact and well balanced; it allowed the water to be carried next to my back. It needed to be packed carefully, but I felt it was worth the hassle.
3. The Sea and Summit bags did a good job of keeping everything dry.
4. I had a light long sleeved top as well as a light fleece; the light top was often better when hiking, when a fleece would have been too warm.
5. In addition to this, my normal hiking clothes were a light tee shirt, trail running shorts, light boots, sun hat, sunglasses and walking poles.
6. The above weight doesn't include food and water for the day. Normally this would add approximately 2 kg for the water and a bit less that a kg for the food.

I don't think you can get things much lighter than this without reducing your margins below what I was comfortable with. The most obvious way of lightening things would be to reduce or eliminate the emergency bivy gear. Another would be to not take the topoguides and instead rely on following the red and white signs (maybe augmented with photos of the maps on a smart phone).  

I wouldn't change much in what I took; everything performed very well. I would have preferred a merino shirt to hike in (they dry more quickly after washing and smell less), but couldn't find one in a light (cooler in the sun), bright (better for safety reasons) color. I would also have taken a long sleeved merino shirt as an alternative to the light top I sometimes wore when hiking. I would consider taking a well stocked Kindle for reading next time.