What glycemic index ranks and what it ignores
Glycemic index (GI) ranks a food by how quickly and how much it raises blood glucose relative to a reference, using a fixed test amount of available carbohydrate. It is useful for comparing the quality of carbohydrate sources, but on its own it says nothing about how much of that food a person actually eats. A food can score high on GI yet contribute little carbohydrate in a normal serving.
This is the gap glycemic load is designed to close.
How glycemic load accounts for portion size
Glycemic load (GL) multiplies a food's glycemic index by the grams of available carbohydrate in a realistic portion, then scales it down. Watermelon is the classic example: its GI is high, but a typical serving holds little carbohydrate because the fruit is mostly water, so its GL is low. GL therefore better reflects the real blood-glucose impact of a normal serving.
This is general educational information, not medical advice. Both metrics are simplifications, and individuals managing blood sugar should plan with a qualified professional.