Seems to me like there's probably something else in that spray because I use hydrogen peroxide on a q-tip to kill mealy bugs and it's never smelled. Perhaps it's the quantity used, but my guess is something else is in the spray. Maybe rinse the plants in water to rid them of the smell?
As for plant gnats I've found a couple of things that worked for me. One is to re-pot the plants using ABG mix (think you can get it through Josh's Frogs) which helps with the drainage in the soil of the plants. Another option, is to buy a Katchy - they have them on Amazon, and it's a small electric "zapper" (not quite right but it's early still) that has a light which will attract the gnats out of the cage and into the trap which uses a sticky pad to catch them there. It works wonderfully, and is silent. If your chams are in a room where you can close the door to keep them safe, you can wait until they are fully asleep, then go in an open their cage doors, and then close the door to that room, allowing the gnats to escape more easily. As long as the Katchy is the only light in the room they will be attracted to it and then get stuck. Good luck!
