Yes, fueling for a marathon is a tough one for folks on keto. May I ask your primary reason for being keto?
If your carb tolerance is high (likely so if you’re training for distance and not insulin resistant), you probably can get away with have more carbs in the days before the race. But I wouldn’t over do it, the point is to just make sure your glycogen (muscle and liver) is at peak. You’ll probably see a few pounds of weight gain when you do this (it’s water and glycogen).
I have to say I’m not a fan of Gu. Have you considered any real food alternatives like a banana or an orange? I know it is not as convenient, but it is much better for you.
I assume you have a last long training run coming up. I’d experiment with your fueling strategy and if you can, test ketones along the way. You’ll learn more by trying something because we are all wired slightly differently.
Let me know how it goes.