I think the gel question didn’t really phrase what the “no early gels” rule was about, because there is some serious advice I’ve seen on this. I came across it on a podcast with Daniel Healey, former Head of Sports Science at Tinkoff Saxo (link).
The theory is that if you have a gel or other high glycaemic sugary carb before or very early on during a ride, it can cause a spike in insulin followed by rebound hypoglycaemia, making you feel tired, lethargic, or perhaps jittery. However once you start riding and do some hard efforts, catecholamines are released which inhibit insulin secretion and protect you from this effect.
That’s the theory anyway. It wasn’t really anything to do with how it affects fat oxidation for the rest of the ride. It may be there are holes in the theory, but for me the credibility of the source suggests it’s more than bro science.