As you rightly say, it very much depends upon the individual as we aren't all the same and our bodies will respond differently even if we eat/drink the same food item.
The Gi/Gl table gives an AVERAGE response to the sugar/glucose of that food/drink item and pure glucose is used as the reference and considered as being 100%. In other words a high Gi/Gl food/drink item will produce a rapid rise in Blood/Glucose levels in a very short time, that's why a glucose drink is given to someone who's blood/glucose is too low and needs to be brought back to a satisfactory level within minutes.
We diabetics in order to minimise LONG TERM HEALTH COMPLICATIONS are advised to try our best at keeping our blood/glucose levels within a 4.0 mmol/L to 7.8 mmol/L all the time (something that's impossible to achieve).
By allowing our average blood/glucose levels (the HbA1c level that you will frequently hear mentioned) to rise above the recommended safe band we are progressively putting our internal body organs at risk, the risk being worsened the higher up the blood/glucose level is. Uncontrolled diabetes can over time cause very serious complications, amputations to limbs, blindness, kidney failure being some of the more serious health issues which are best avoided in the first instant wherever possible.
The occasional piece of pie, ice cream or sweet treat, though it will raise your blood/glucose level, shouldn't do you so much harm that you would need to worry unduly over this,. we all break the diabetic rules from time to time.
Incidentally symptoms don't always appear and aren't obvious. In my own case I went 10 years before my diabetes was finally confirmed. The HbA1c test should help in your diagnosis, but it can often appear that the individual has diabetes when the cause is in effect the result of something else i:e: Thyroid or hormonal imbalances. You will need a proper full blood test done or maybe a OGTT before being diagnosed as diabetic.
Best wishes - I hope that this reply answer's your questions.