I'd tend to agree with ronnie suburban's definition of "gourmet." However, like so many of those who have responded to your question, I am much more likely to shop at a market with a multi-ethnic clientele, where I can get "gourmet" ingredients at significantly lower prices. (Marketplace on Oakton shelves their their pantry products by country of origin.)
To me, a "gourmet" shop (Fox & Obel) offers a narrowly edited selection of produce and meat in comparison to a general market with a multi-ethnic clientele. I don't recall seeing pigs trotters or tongue in the meat cases there, for instance, and durian, a regular at H-Mart, is only occasionally seen at F&O.)
A "gourmet" store also offers more prepared foods. There is at least a perception that rather than mass-market salads prepared in factories, they tend to make more dishes in-house. Some of these dishes are things that only the domestically impaired would consider purchasing ready-made.
Another thing that characterizes a "gourmet" shop is the profusion of tiny artisanal brands. While European products are well-represented, many of these are American products like gourmet potato chips or small-batch jellies. There is an emphasis on the packaging that tends not to be present to the same extent at the general multi-ethnic markets.
Another possibility is that the label "gourmet" is a function of the address and physical characteristics of the store. I think of F&O as Chicago's Fauchon or Harrod's.
Historically, (since the earliest days of restaurants in the U. S.) all things French are considered gourmet in this country, and a true gourmet store tends to offer items like foie gras (I know, I know) that are taken for granted in France, but rarely available here. Currently, Italian specialties threaten to displace the French ones in the affections of Americans.
Whole Foods has been mentioned above. The reason I go to Whole Foods is to purchase organic and all-natural items like Tom's Toothpaste that I can't find elsewhere. I grudgingly purchase other items, for the convenience, as long as I am there.
One spot that has not been mentioned in this thread is Treasure Island. Though they are certainly not cutting-edge in either the general market or "gourmet" dimensions, I imagine that they might have been at one time. (They should do some focus-groups to re-tool their stores, I think.)
This sounds like an interesting project. Good luck with it.
Man : I can't understand how a poet like you can eat that stuff.
T. S. Eliot: Ah, but you're not a poet.