I'm not sure I'd call it a "gift" so much as a sensory sensitivity, which can be a gift and a curse at the same time.
My DD8 has detected small changes in home recipes... for example, she loves DW's rice pilaf, but has refused it when it was made without onions.
She has also famously refused to eat restaurant food that, based on all our previous experiences, she should love. Whoever ate of that dish later exhibited the symptoms of mild food poisoning. This has happened too many times to be chalked up to coincidence.
I'd definitely say your DD is gifted with an ability to sense taste! Of course onion is a strong taste in food usually and maybe not what most would consider an extraordinary "pick-up", but the food poisoning anecdote is beyond the norm, Dude.
I would call that level of sensitivity a gift.
