If you want to know more about it you can just read about how diabetics treat different foods. Short version; high GI is fast high blood sugar, lots fat and sugar can results long period of with high blood sugars.

It is a CGM more or less what you would expect, insulin is the only hormone that lowers blood sugar, then you have all the stress hormones that raise it like adrenalin, growth hormone and cortisol plus glucagon for long time storage of sugars in the body. So measuring blood sugar with a CGM gives you a value between 2.9-13.5 mmol/liter that is supposed to give you picture of a rather complex system. A CGM also needs perfect access to your free flowing blood which in it self is a difficult task.