If you have a sport drink and a gel, if your stomach draws out the water you get left with a concentrated sludge of flavor glop. I can tell you from personal experience IT IS THE WORST!!!!!! It can make the 24 hour flu be like a walk in the park.
You need the carbs, etc. for the long run. Use the Gel and stoke up on the water. Have the recovery drinks ready when you are done. If you like the sport drink, use one designed for athletes, not the convenience store. Accerelade is pretty good.
I agree that GU plus a sports drink = a gut bomb! I do use GU and sports drink during a marathon, but NEVER at the same time. I have water at the station before and after I GU and wash down my GU with water only. Train with GU and see what you think. The very diluted sports drinks on a race course are not concentrated enough to give me what I need on race day thus, I tend to get more out of GU. Hope this helps good luck at Boston!!
The combination of GU and sports drink doesn't usually agree with most people. Plus it provides more carbs than you really need at one time.
Some people like to alternate the two. Always try it in training first.
My preference is GU and plain water. If it is really hot I also use electrolyte capsules. PC
Everyone's different, you need to do a bit of personal experimentation. But I agree with consensus: sportdrink andGu is way too much.
During runs > 15 miles, I do gel and water. Gatorade before and after.