Assuming you have read the reports of my shortened version of this hike, below you can find my findings on the GR738.
Hard to say if you have not done the whole hike and on top of that seen little of the landscape due to low hanging clouds. But I know I will be back. On the one hand because I know that there are beautiful landscapes to be admired here in Belledonne and on the other hand because the little I did see made my mouth water. And the mix of steep, almost off the beaten track paths with numerous mountain lakes, views and peace and quiet really appeals to me. So, I will be back and preferably as soon as possible!
The GR738 is physically one of the toughest GRs there is. In ascent per kilometer it is more difficult than the infamous GR20! I only hiked about 50 km of the total 130 km of the GR, but I can tell you this: it's tough. If you want to do the GR in five or six days, you have to be physically very fit. Even if you do it in six days, you still have to do an average of 1500 positive metres every day. So those are tough stages. And supplies also play an important role (see below). Many do it in ten days or so.
Apart from your physical condition, this GR requires you to think about your food plan. The first manned refuge is only after 62 kilometres and is even situated half a kilometre from the trailhead. About ten kilometres further on you'll find the Gîte of La Martinette where you can buy not only good food, but also some groceries (like pasta, bread,...), but the supply is very limited. After that there are three manned refuges: after 90 km (Habert d'Aiguebelle), after 98 km (refuge Jean Collet) and after 104 km (refuge de la Pra). Keep in mind that due to the physical nature of this GR, the distances in a day can be very short and that you therefore have to bring more food than on other hikes.
Nothing to complain about. Just follow the typical white-red markings. However, at the starting point in Aiguebelle I only saw them from the tourist office and not from the station.
Best time of year
Between the end of June and mid September. There can still be snow very late in the year and this can cause extra difficulties on some passages. Crampons may be needed on the steepest sections. During a weekend in August, the Echappée Belle is run. This trail running event goes from Vizille to Aiguebelle (south to north) and takes in many parts of the GR738. It's best to avoid the GR route during that weekend.