You should really practice nutrition on your longest long runs. "Practicing" nutrition on a relatively short long run like 12 miles isn't going to tell you much about how you're going to feel nutrition-wise in the last six miles of a marathon. So... I don't have any suggestions for you for this race, but maybe for next time you can try switching to something besides gu and see if you like that better. My stomach is also a big concern for me, and I tried out a variety of different things before settling on a drink mix that I tolerate well. I think drinking nutrients (rather than eating gus and such) helped me, as well as allowed me to pace my nutrient intake better rather than trying to down a gu or half gu at once.

It sounds like you've run marathons before, so why not use your times for those to make your pace plans?

I'm confused about what you mean by "positive split" for the last 6.2 miles. A "positive split" would mean that you're running the first half of the race FASTER than the last half of the race. This is not a good idea - it usually leads to complete disaster in the second half and an overall poorer performance and experience. Frankly, by the last 6.2 miles, I can never do much more than hang on for dear life. If it were me, I would likely start out 60-30 sec slower than my "goal" pace for at least the first 6 miles, see how I feel, and ease up to get progressively a bit faster from there, provided I'm feeling good.

Certainly conventional wisdom is to use some, or all (opinions vary) of your 18+ mile runs to be practicing your nutrition. That isn't an option for you right now, but you have a marathon history to draw from which is helpful.

Could your stomach issues be from the Gu? Like most gels, Gu is not isotonic so does need you to be taking in the right amount of water, else you are going to struggle to absorb the carbohydrates you are consuming, leaving you with less energy and your stomach struggling to process. For a regular Gu, you need 175ml water to make it isotonic. 200 for a Gu Roctane (source: https://fellrnr.com/wiki/Comparison_of_Energy_Gels)

I personally like SIS Go Gels, because they are isotonic. The downside is that they are therefore larger and some find them bulky.

I'm not sure the value of sharing my own experiences, since we are all different, but here goes.....

My first 20mile run in this current marathon cycle did not go well. I maintained the pace I was aiming for, but I expected to finish the run feeling like I still had energy in the tank and I didn't - I was shot. It really hit my confidence. I'd been experimenting with more nutrition than in the past and so I expect to feel like I had more energy, not less.

I realised I had miscalculated the amount I was consuming and was taking in more than I intended. 3 weeks later I scaled back to 3 x SiS gels per hour (giving me a total of 66 carbs per hour), in line with the general guidance that 60g of glucose per hour is about the limit of what the body can process (in other words, 1gram per minute). Every third gel was one with caffeine. I also upped the water intake from 300ml to 500ml per hour. The second 20mile run was way better. I held the same pace throughout and finished confident I could have continued that pace for another 6 miles.

For my final 20 miles, I'm going to experiment by adding some fructose, which is absorbed by a different channel and so you can add on another 35-40g of carbs to the 60 I'm getting from the SiS gels. I'm not a quick runner, I'm going to be out there on race day for probably 3h 50m, so I'm going to want all the energy I can get

For your pacing question, I'd concur with others here - aim for negative or even splits. Personally I aim for even. You might not achieve it, but it's the best approach. In my limited experience, the mental boost I got from overtaking hundreds of people from mile 20, more than made up for feeling like a tortoise at the start.

For context - I'm not a marathon expert. One marathon behind me, my second coming up in April and simultaneously training for an Ironman 70.3. I've got a triathlon coach working with me and have been researching and experimenting with nutrition quite a bit these past few months during long rides and runs.

I usually go for a banana half an hour before the race, then a Gu every 4-6 miles. I drink Gatorade at every other water stop, but not the same ones where I take the Gu. My last marathon, I stopped too soon, because my stomach was feeling a bit wonky. I should have continued.

I agree about trying for even splits. I always go out too fast and slow down at the end. Usually my too fast and too slow even out, so I end up about where I wanted to be, but I do struggle at the end. Everyone tells me that I'd finish stronger if I slowed down more in the early miles.

For my long runs I put my water out before my run (maybe the night before - depends on temperatures and my schedule). I have an out and back route so I hit up each of my bottles twice. I eat Cliff Bloks at roughly 200 calories/hour and stay away from caffeine (although I think I'm going to try it). I'd LOVE a negative split... I too am going to work toward this.