@penn. I found this explanation, but there is so much more! Examples, people who do it, and more. Just google “gaslighting”.
Gaslighting is a colloquialism, loosely defined as manipulating someone into questioning their own perception of reality. The expression, which derives from the title of the 1944 film Gaslight, became popular in the mid-2010s. Merriam Webster cites deception of one's memory, perception of reality, or mental stability.

I think in the case of medical patients, it refers to the long history of countering any complaints for which they have little understanding with trying to convince us that our conditions are due to some psychological problem (especially if a female patient), attention seeking, drug seeking, hypochondria, “lifestyle choices”, and so on. Doctors have dismissed as quacks integrative health testing for such things as gut biome imbalance, gluten intolerance, and acupuncture as new age nonsense, only to be proven wrong time and time again and even to eventually have it confirmed by research, and then to embrace and recommend such supposed quack treatments decades later. Western Medicine in general finds it easier to throw pills at patients in order to dull symptoms rather than to take the time and make the effort to investigate the root cause of problems.
Examples are gluten intolerance, yeast and mold sensitivity, leaky gut, endometriosis, chronic fatigue, Lyme disease, fibromyalgia, IBS, bacterial stomach ulcers, hypothyroidism, etc., but remember doctors also originally rejected as preposterous the idea of germs, hand washing, the usefulness of vaccines, and they used to smoke while working in the hospital.
They also (generally) have the grave fault of not seeming to understand statistics. If the majority of patients diagnosed shared certain symptoms it does not mean 100% of them present typically. Trying to convince patients that they are not feeling what they are feeling, or that they don’t know what is unusual for their own bodies is gaslighting.