The 75-Hard Challenge mixed with GLP-1

Everything else seems workable but the gallon of water requirement is pointless. Someone just picked a random amount without considering it might not suit everyone and could even cause harm. Especially when coffee, tea, flavored water, fruit, and soups are disallowed. It risks throwing off electrolytes. I've considered trying 75 Hard but I'm skipping the gallon part. I'd choose a sensible daily amount and tweak it based on how active I am that day.

Endurance seems to go up when using GLPs because the body shifts to burning fat for fuel. Make sure you have enough supply for the full 75 days. There are other peptides that might help with energy as well.

This should be doable. Eating a small piece of a protein bar, chicken, or cheese each time you drink water can add the needed calories if you plan it out. You'll likely need a calorie surplus because the GLP, especially Reta, already increases calorie burn. Is adding something like Naked Mass to the water allowed? That would cover the carb and calorie needs for keeping energy up during workouts.

Let us know how it goes. Since I started Reta I've found it very difficult to get my water in.

It depends how far along you are with weight loss. If you still have a lot to drop then the fatigue and appetite suppression are normal. You could lower the dose until you hit the right balance of suppression without losing too much energy, especially if you're already at maintenance. The gallon of water is possible but you need to cut back on other drinks. Ask yourself what your actual goal is with the challenge and keep asking why until you reach the real reason.