I'm not sure that it's as simple as some posters are suggesting. On one level, I agree that amateur diagnoses are often irritating. In my lifetime, I have seen trends come and go - 'She's obsessed with . . .', 'He's a bit OCD', 'X is neurotic' etc.
The current fashion is to label every sort of bad behaviour as gaslighting. I think that this is unhelpful, as people who are genuinely being gaslighted must feel that it is trivialised, and it probably makes those who are told about it think that it is not too serious, when in fact it can be very destructive.
Similarly, narcissism seems to be a fashionable accusation, and it is, of course, linked to gaslighting, in that narcissists often use gaslighting to undermine their victims. OTOH, as someone said upthread, if you are the victim of a narcissist seeing a description of their MO can be a revelation. It is such a traumatic thing to go through that it can be difficult not to blame yourself for everything that happens in the relationship (whether that is a 'romantic' one, a parent/child dynamic, a work relationship or any dynamic between two people).
Typically, they 'love bomb', which can take many forms, but ultimately means that they make you feel that one way or another you can't manage without them. It is insidious, and happens to people who are otherwise strong and capable. The next stage is to 'devalue' you, which they do by chipping away at things you hold dear, until you lose all sense of who 'you' really are. This is confusing, and you think that it must be you who is at fault, as after all, you are their right hand woman, or their loved one, or their daughter, and you internalise the blame. They will use other people to reinforce your feelings of worthlessness - 'Everyone says', or they ignore what you say and then attribute it to another person as though you don't matter.
Every now and then, they are kind to you, and you get the dopamine hit you used to get when they were love-bombing. It's like an alcoholic having 'just one drink' - before you know it you are hooked again. The final stage is the discard, when they move on, and you are trying to get back in their good books, confused and miserable.
The way in which this happens varies according to the situation and the players, but the basics are the same. Seeing this in a book or online can be a revelation, and suddenly it all makes sense. Knowing that so many other people have been victim to this sort of behaviour can be oddly comforting, and the advice (to cut all contact) can be really useful.
Narcissism really isn't about conceit about appearance (although that can feature, as it can with anyone). It is also about real abuse, about a need to have people make them feel 'special' and 'superior'. I do understand why people who have been through this abuse feel entitled to use the term, whether or not they are qualified psychologists. I don't need a dentist to tell me when I have toothache.