From wikipedia, "glycaemic index (GI) is a number associated with a particular type of food that indicates the food's effect on a person's blood glucose."
Because glucose is a sugar, you seem to be confusing it with fructose (another sugar, 5-carbon vs 6-carbon). The glycaemic index specifically looks at one specific sugar -> glucose. Sugars aren't intrinsically bad, but often times they are removed from their origins which contain other constituents that help our bodies digest them (all sugar, no pulp).
Because glucose is a sugar, you seem to be confusing it with fructose (another sugar, 5-carbon vs 6-carbon). The glycaemic index specifically looks at one specific sugar -> glucose. Sugars aren't intrinsically bad, but often times they are removed from their origins which contain other constituents that help our bodies digest them (all sugar, no pulp).