Nutrition problem - only AFTER 6 hours

I think my stomach can only handle energy gel for about 6 hours. I have done several endurance and ultra events. Often my stomach is the problem in the later stage and I get incredibly nauseous. I've tried to eliminate the variables such as going out too hard, etc. I think the one constant is that in the events where my stomach shut down, I had fueled ONLY ON ENERGY GEL. (IM Austria, AR50 2012, AR50 2013). Yesterday, during the AR50, my stomach shut down at mile 40, and eventually everything came up by mile 48. Unfortunately, I couldn't start replenishing since I still felt nauseous.

In IM AZ, I used Infinit exclusively on the bike. And on the run used 90% gel, but, again, that was for less than 6 hours. And during a double century, I used a combination of things.

I have had trouble with liquid fuel like Infinit during cooler months since I just can't seem to force myself to drink in enough of it. So I've faded during longer workouts.

I know each person's stomach is different. Mine definitely is on the more sensitive side. That's why I've shied away from using solid foods.

I've done lots of half IM and am fine with gel only, but those take less than 6 hours.

Have any of you had similar experiences? If so, what changes worked for you?

Thanks.

Granted Dr Lim's approach is a bit on the heavier side since they can carry everything in a car and just hand it to a rider as need be. I think the general principles still apply.

https://www.youtube.com/watch?v=nGTu8yJesmE.

Do you have this issue with just one brand of gel or many types? I find that some are easier on my stomach than others. Have you tried multiple brands, e.g. GU, PowerGel, Clif Shots, Hammer Gel, EFS Gel?