In addition to the link in someone else's comment, it's part of the nutrition strategy from this book, which has helped me greatly.
Really late reply, but in training my LRs are generally mostly 24-milers. For early in the cycle, especially in cooler weather, I'm just running easy-ish, so I generally don't bring any nutrition. For later in the cycle, I'm usually doing TLTs or a significant number of MP miles, and take 3-5 gels throughout the run, evenly spaced, usually without water because I'm too lazy to carry gels *and* water.
For races I use this book, which (among other things) recommends 60-90 g carbs/hour while racing. I prefer water to Gatorade in races, so this works out to 9 gels during the race: one a couple of minutes before the gun goes off, and then another every 3 miles. Ngl, it's a lot, and if the weather is warm I'll back off a bit due to limitations on the amount of fluid available. It's ok to have some stomach discomfort during the race - part of the point of taking gels in training runs is to get your GI system used to it (the other part, of course, is to help nail the workout).
Grandma's is a great race - I've never done, but at least a half-dozen members of my club are going this year. Good luck!
I've heard of fat-loading for 3-10 days, followed by carb loading, for a race. Never heard of low-carb as a long-term training strategy. I use this book as my nutrition bible for running, and have not bonked since in a marathon, while improving my times by over 15 minutes.