That still wouldn't be enough water for this trip, though. You'd need double that, plus electrolytes. And, with all that weight, the trip is not logistically possible IMO. The weight would be very problematic in that heat. Even carrying 3 liters can feel like a burden: it weighs 6 lbs approx without the container.
IMO a hiker in those conditions wouldn't be surviving until dark. I believe heatstroke death comes quickly.
That particular pack (Juno) has very high reviews. Very nice one IME.
Yeah. If you were planning on being out there for 4 hours in 100+ heat, 2 gallons of water & electrolyes would not be too much. That water is going to weigh about 17 pounds, so even though you are consuming it, it is a dis-incentive to carry more safety supplies or gear, and you are just adding huge additional stress to your cardiovascular system.
And even under shade, that temperature is very stressful as the ambient air is not cooling your body at all. Add direct sunshine and 100 will feel like 110