Isn't гастроном a grocery store, where you buy food?
It is, but they are almost extinct beasts.
For whole giant Omsk just few has left:
гастроном 2 гис.png
They ceased altogether with USSR. The former shops turned into banks, travel agencies, and boutiques. Instead, "new russians" bought 1st floor flats in multi-apartment buildings and started renting tiny parts of them to individuals. These "businessmen" in turn employ "shop woman" which for $200 a month will stand there without holidays since dawn till dusk. One shop woman sells only one category of food: for example, sausages and stuffing (100s sorts of it, but all are the same). From outside it looks like:
Бутик.jpg
From inside - you don't want to see it.
These "points of sale" could be called "магазин" or "мини-рынок" and often have own name. The owners of real estate like to call them "супермаркет". So, instead "Вовка, сбегай в гастроном за макаронами" now we say "Вовка, сбегай в "Апельсин"("Корзину", "Олимпию", ...) за макаронами"

Normal Гастроном should be large enough to sell main products, has clear large shop windows and they used to be looking like this:Гастроном 3.jpg
So, Гастрономы are another subject to feel nostalgic. Реклама гастронома в Karlovy Vary, Czech rep.:
Гастроном 2.jpg

BTW,
most "news" in local media are "in future tense". So, this Гастроном
В Омске откроют гастроном для обеспеченных покупателей | ОБЩЕСТВО: Транспорт | ОБЩЕСТВО | АиФ Омск
has never been opened.