Sensitivity to differences in wines differs widely. Partly, I think it's a matter of genetics, partly a matter of training and experience. If you have no reference points it's hard to make comparisons. I'm perfectly prepared to accept that he can't tell a $40 wine from a $6 wine, just as I accept that some people have perfect pitch and others can't tell one tune from another. General Grant was (in)famous for having said (listening to a band) the tunes all sounded like Yankee Doodle to him. I do think that most people, who want to, can improve their palates and can learn to differentiate among wines on a quality basis, if not a price basis - I'd be the first to agree that while price is a general guide to quality, it's not a reliable guide, and there is a whole lot of wine sold for premium prices that is probably worse than many sound commercial wines (like your Gallo Cabernet - a poor replacement for the blend that was Hearty Burgundy back in the day) costing a half as much or less.
I have an eight word theory of wine tasting:
Pull Lots of Corks
Remember What You Taste
The last four words are important to figuring out what you like and being able to replicate it other than by looking at the label. Everyone has some sort of taste memory, but few people make a point of training it. (just as few people train their memories much at all, these days). Your taste memory includes '70 La Mission, which is a good wine in a very fine year - an excellent benchmark. My benchmarks are different for Cabernet, though several 1970 Bordeaux and California Cabs are among them. And, I'm lucky enough to have tasted many of these wines through their life cycles.
It has become harder to find daily drinkers - my base point is around $15 minimum these days, though I can sometimes get a case of something I like for ~$10-12. For special occasions, for dinner parties, and even for a 'special' dinner at home, I go to my cellar, where I have wines I bought young, and have kept until maturity (or I lose my patience). I rarely drink Cabernet under the age of 10, prefer my Zin between 8-15, and other wines at varying ages. YMMV.
I have pulled a few corks in my time, and have learned that I consistently receive greatest pleasure and value from the southern Rhone valley: cotes du Rhone, Chateauneuf, and Gigondas go very well with Louisiana food. I’ll drink a GSM from Australia also and have enjoyed some immensely. But these are $15-$75 bottles, and that’s not my everyday price point. Thus my plea for plonk.