Aside from discussions on why you would want to do it.
Obviously, you will need to tap into "gray area", but the question is what gray area foods.
I am currently thinking oats (real ones) and red lentils. Both are convenient (quick to cook), low GI and high in carb. What do you think?
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I wouldn't recommend it, but unsweetened apple juice has a GI of 40 or so, where a jug could get you up over 200 grams before you've eaten any food. Fructose has a glycemic index of 19.
The easiest way to do it is to blend foods. All foods are nutrient blends, and by increasing their fat or protein content you drop their GI. Want to drop the GI of a sweet potato? Dump a can of tuna on top.
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How about green plantains. If you fry them they have a GI of around 40 and they almost taste like bread. When I am "carbing" they are my go to source. Mix 2 eggs, 1 plantain, 1 tbsp coconut oil, and lemon/lime juice in a blender then fry them into pancakes/tortillas. Awesome for fish tacos.
"How would you get massive carbs without going higher than 50 (at most 60) on GI?"
If you want 'low GI' foods, just eat your carbs with 'other stuff' (see Jaminet post linked below). The Glycemic Index is just an index of 'standalone' foods.
Jaminet wrote a related post called 'How to Minimize Hyperglycemic Toxicity' which covers reducing the GI of foods. Though i think it is better described as lowering the glycemic load (GL), as the foods are not eaten alone.