Gatorade and energy packs etc are really meant for long-endurance sports, not short-term high intensity burst sports like hockey and are really nothing but sugar with some salts. It takes 30 minutes+ before anything hits your bloodstream, so by the time you've drunk it or eaten anything, it's too late. I drink flavoured water, only because I don't like plain water during a game, even then mine's usually a few sips and that's it.. If you notice many of the nhl players spit their water / gatorade out..
If you're gassed at the beginning of a game, it might be because you've eaten too much, too close to the game - I've noticed this hugely if I eat and then play within an hour or 2 of a game, I feel like I'm skating with weights on. Digesting food takes a lot of energy (that's why you fall asleep after eating Christmas or Thanksgiving dinner, because of lots of calories and nothing to do with anything like tryptophan). I play 4-5 times a week, and I'll eat a fairly light meal minimum 3 hours before a game. On the way to a game, then I'll grab a drip or regular coffee and a banana. I keep track of HR and I've found now that I can play pretty intensely right up until the final whistle by playing more tactically in the first period, ramping it up in the second, and then going hard in the third.. Quite a few times I'm sharing centre, so playing 50% of the game and lots of ice.
It might be best to keep a log of what you did, and how you feel during a game..