Quite a bit happens in the scene in which Gil saves Zelda, including the important moment when Adriana discovers that Gil is engaged. There is one other point that slips past, as Gil gives Zelda a Valium™ and attempts to explain what it is. It is, of course, medicine from the future, a drug completely unknown in the Golden Age and common in the Modern Age. As he is explaining this, he begins to recognize the implications of being in the past.
We know he recognizes them, because when he is in La Belle Epoque with Adriana it coalesces, that to stay in La Belle Epoque means to live in a time without antibiotics, and so too to stay in the Golden Age is to lose all of modern pharmacology and modern medicine. The past sanitized is a wonderful place; the past actualized is terrible. Life expectancy increased more in the twentieth century than in all prior recorded history, due primarily to advances in medicine and sanitation and nutrition. Life is always hard for most people, no matter when or where you live, partly because real equality is impossible: some will always have better lives than others, always on a bell curve such that most people know that there are others who live better than they do. Everyone in the United States in 2010 lives better than anyone in the United States in 1890, simply because of the advances we have made. The Vanderbilts and the Carnegies and the Rockefellers and other capitalist moguls had things we do not, but those did not include air conditioning, modern medicine, sewage treatment, not to mention television, computers, air travel, synthetic clothing, and plastics. That Valium™ was Gil’s first glimpse at the real wealth of the future and relative poverty of the past.
It probably is not serious otherwise, though. The fact that Zelda took one dose of a future psychiatric medication at a time when she was upset, and it helped her, might encourage her to see a psychiatrist, but psychiatric medicine in her time has nothing comparable and she will undoubtedly be disappointed. We might think she would devote some of her fortune toward the development of better medicines, but she never had much success as herself (and thus has no personal fortune) and her husband did not see much financial success after The Great Gatsby, so it’s not as if there will be tremendous impact from this. This also predates the age of celebrity spokespersons (it begins on a small scale with radio but does not come into its own until television). She might like to see the development of more Valium™, but she is not going to be able to cause it.
So ultimately, when Gil gave the Valium™ to Zelda, it had more impact on him than it did on history.