It's a complex problem at the grocer level, as you are appreciating.
Having a bunch of Tallegio cheese on hand is likely one of those things that does not actually pull much/any profit for the store, but having a good selection of perishables attracts the type of people in the door that care about that. So if they lose a bit by regularly carrying dragonfruit or something, they need to make it up elsewhere. With tens of thousands of products in the store, it ends up being complicated and I am sure you can find some items that are marked up way more than others, and some items that they may even regularly lose money on. I'm certainly not a grocer, but it doesn't surprise me that it all evens out to some pretty thin margins. Is setting a price on some items to be higher than others in order to offset losses "price gouging"?
Or, as others have pointed out, does selling fresh baked bread for $4/loaf and yesterday's bread for $1/loaf count as "price gouging"? Dynamic pricing isn't always nefarious, even on non-perishables. If a new blender model is out, you might heavily discount whatever older model stock you have left just to cut losses and get rid of it to clear up the space for the new version/product. Did the purchaser who bought the blender the day before it went on sale for 50% off get "gouged"? I don't think so. The devil is in the details and how you define it.
I agree you are much more likely to find "price gouging" at the level of individual industries or companies. Collusion on setting prices and things like that are always a thing, and are constantly getting investigated. Not going to point out a particular industry because I have no idea, but that's why we have anti-collusion and anti-monopoly laws.
Harris is pandering here by selling a simple solution to a complex problem because she thinks it will be popular. She's a politician, it's unfortunately what they do. They've been trying to sell simple solutions to get elected for so long that half the public thinks all problems actually are very simple and that the politicians are just idiots or even nefarious for not having it all worked out. The real question is which politicians actually believe their own BS and will try to implement it, and which know they are selling snake oil on certain issues and might make a show of trying to prod Congress on something, but realize it will quietly die in committee and are content to leave it at that and say "I tried". I know exactly which type of politician Trump is. I think he actually believes he is a super-genius and all problems are simple and others are just too stupid to realize it. For Harris, it remains to be seen.
See also "No taxes on tips" which both candidates have endorsed. That sounds nice, but is going to be a bear to actually implement in the intended way because it is far more complex than either candidate is letting on.
