What's wrong with insulin spikes the way you say it, is that while insulin is the most anabolic hormone in our body, remember that anabolism doesn't only mean muscle growth, it can mean adipose (fat) storage as well. Insulin mobilises fat as well as protein.
A meal containing a large quantity of high-GI carbohydrates triggers a large release of glucose into our blood stream, along with increased insulin secretion (the insulin spike), which in turn means a rapid uptake of the glucose, which either goes into energy stores first (in our muscles) and the liver as liver glycogen, but any excess is used in conjuction with the insulin to form fatty acids in the blood, which eventually go into our adipose stores.
More simply, in most cases our body can't use the excess glycogen in the blood stream and will put it into adipose storage, in the same way as excess calories also go into fat. This is why you can be in a calorific deficit but still gain bodyfat by eating a bad, high-GI carb diet.
Lower GI foods mean that the release of glucose into the bloodstream is much slower, which in turn means that less insulin is secreted, so excesses and therefore fat storage is minimal.
A lot depends on the individual as to how many high GI carbs they can eat without gaining fat thanks to the insulin spike, if you want to see how much you can handle base your diet around high-GI carbs like coke, white bread, white rice, and the like - but this in turn will play havoc with your energy levels and blood sugar. Your sudden drops in blood sugar will leave you feeling tired and spent for a lot of the day, and won't help during the workout either. What gets you through a workout better, a big bowl of oatmeal or a round of white bread jam sandwichs?
You'll be constantly hungry so you'll be eating all the time, yes, but the likelihood of fat storage is greater. If your metabolism is that fast then it might not make a difference but I wouldn't recommend you used high-GI carb sources.
Like I said, this is more or less the science behind it, my own understanding is amateur and I'm not a personal trainer.
If you want to be able to eat more, go for a little walk before breakfast to stimulate your appetite (not a cardio sessions as such, just something to get your heart going).
If you want more calories in your diet, make them protein calories or essential fats, too many carbs will have the negative effects on adipose as typed above. (and a too large amount of low-GI carbs can also create an insulin spike by the way, as can a large amount of fat)
If you think you're a hardgainer (I bet you're not eating enough anyway), aim for 2g of protein per lb target bodyweight first, if you're not gaining after that then add more fats too (olive oil, flax oil, peanut butter, seeds, etc). Remember to keep your meals pro/fat and pro/carb because the insulin release associated with any sort of carb makes storage of dietary fat as adipose much more likely.
Carbs should be a last resort to increase if you're not gaining.
ps - in my experience, true hardgainers are hungry ALL the time and don't have a problem with eating constantly.. if you're NOT always hungry then you're unlikely to be a hardgainer, it's all in your head.. check your basic diet and training.
