What is the origin of the phrase "Gag me with a spoon"?

How does a phrase such as "Gag me with a spoon" originate? I understand the sentiment as a real concept — gagging oneself with a spoon causes a choking sensation without actually constricting breathing. It's uncomfortable and disconcerting.

But, how did this become the go-to dismissive phrase of the 80s?

The phrase "gag me with a spoon" originates from Valley girl speak, which is in turn an obnoxious form of Californian english. From the wiki article:

Valley Girl (or Val, Val Gal) is a stereotype leveled at a socio-economic and ethnic class of American women who can be described as colloquial English-speaking, materialistic, self-centered, hedonistic, and often sexually promiscuous. Valspeak is also a form of this trait, based on an exaggerated version of '80s California English.

The phrase is used as an expression of disgust. A typical usage from the online slang dictionary

Oh my gosh, gag me with a spoon. That were the ugliest shoes I've ever seen!

The phrase also appears in Frank Zappa's song, Valley girl.

.../Its like grody.../ Grody to the max/ Im sure/ Its like really nauseating/ Like barf out/ Gag me with a spoon/ Gross/ I am sure/ Totally...

As for the how, you could perhaps reason that the act of gagging yourself with a spoon is pretty disgusting and hence it makes sense. However, I'm not sure you should look for a reason/proper etymology for any of the valley girl-isms (ummm, like, totally, so bitchin', whatever, etc.).

The origins of the phrase is derived from the practice of forcing oneself to vomit, a typical trait of bulimia nervosa. The sufferer will often use their own fingers or a long narrow instrument to induce vomiting; e.g. a spoon handle.

How to Identify Symptoms of Bulimia
Look for evidence of laxatives in your home. Many teens purge through the help of laxative abuse. They also purge by sticking their fingers or other objects down their throat. Look for the presence of spoons or other objects that don't belong in a bathroom for signs of this.

Borderline Personality Disorder: New Reasons for Hope
^{By Francis Mark Mondimore, Patrick Kelly}

The hallmark of bulimia is binge eating, during which persons eat enormous amounts of food, often high-calorie foods, over a period of an hour or so [...]When a person who has bulimia cannot force another bite, she is overcome with disgust and shame at what she has done and seeks a way to rid herself of the calorie load. the vast majority of persons ... do this by forcing themselves to vomit. [...] Individuals with bulimia usually stimulate their gag reflex by putting a finger or spoon down the throat in the early stages of their disorder but often become adept at vomiting at will.

Despite the Ngram's chart's display which suggests the phrase predates the 80s, the earliest recorded usage of this Valleyspeak I could find were dated 1980 and 1981 respectively.

Air Force Driver, Volumes 14-16

Like barfme out; gag me with a spoon. “Oh migosh! I'm getting a used four-wheel-drive. That's going to be like really, like a lot of money - I'm freakin' out, I'm sure. But, I don't know; like it's going to be like so cool, you know. Like see my 4X4.

and The William and Mary Review, Volumes 19-22

"I love you," said Justin Oligarchy "Gag me with a spoon," said Eunice Babel absently, looking again to Joseph Beaumont Darrow IV Two fire trucks roared up to the Auditorium, their sirens wailing. "What's the trouble here?"

A very well written piece charting and explaining the "rise and fall" of this expression, forever linked to the 80s, is in the article Gag Me With a Spoon: “Val-Speak” Takes Over SoCal by Jennifer Ouellette

Frank Zappa’s quirky Top 40 hit from 1982, “Valley Girl,” doesn’t get much radio airplay these days, even on the “nostalgia” stations, but it captured the birth of a linguistic phenomenon in the running commentary provided by his daughter, Moon Unit — “val-speak,” a strange dialect spoken by the female denizens of the San Fernando valley area in Southern California characterized by ending sentences with a slight rise in pitch, as if asking a question, now known as “uptalk.” And it might have stayed in the Valley, too, if it weren’t for that meddling musician and his daughter, who broadcast it to the masses. Soon everyone was walking around declaring their school lunches were “grody to the max,” I mean, “gag me with a spoon,” every uptalking utterance punctuated by frequent insertions of “like” and “totally.”

Source: Scientific American.com