It's from a song "Valley Girl" by Moon Unit Zappa. "Gag me with a spoon" was an expression that "Valley Girls" used. "Valley Girls" are girls from San Fernando Valley in California. http://en.wikipedia.org/wiki/Valley_girl

Ok, I am old enough to remember that stupid statement and I have to say that I am ashamed. Because it came from the Hollywood District, or at least an area of CA that I am not admitted to, I cannot say for sure; however, if one grows up in that area, he or she is likely to have had a "silver spoon" with which he/she was fed with, and the "gag me" expression would have likely emerged from the parent taking in something that shocked the parent while trying to feed the little one with the silver spoon; ergo, gagged the child with the spoon. That's the best I can do! Hope that helps!

Valspeak is a common name for a now partially universal American sociolect, originally of Southern Californians, in particular valley girls. This stereotype originated in the 1970s, but was at its peak in the 1980s and still popular in the 1990s and 2000s. Though for a brief period a national fad, many phrases and elements of Valspeak, along with surfer slang and skateboarding slang, are stable elements of the California English dialect lexicon, and in some cases wider American English (such as the widespread use of "like" as conversational filler). Elements of valspeak can now be found virtually everywhere English is spoken, particularly among young native English speakers.[1] The term "Valley Girl" and the Valley manner of speech was given a wider circulation with the release of a hit single by Frank Zappa entitled "Valley Girl," on which Moon Zappa, Frank's fourteen-year-old daughter, delivered a monologue of meaningless phrases in "valspeak" behind the music. This song, Frank Zappa's only Top 40 hit in the United States, popularized phrases such as "grody to the max". Some of the terms used by Moon were not actually Valley phrases, but were surfer terms instead (such as "tubular" and "gnarly"). But due to the song's popularity, some of the surfer phrases actually entered the speech of real Valley teens after this point. The Los Angeles surfing subculture, on the other hand, did not generally begin using the Valley terms. Valspeak is used heavily in the 1995 film Clueless and quite a lot in Wayne's World. source: http://en.wikipedia.org/wiki/Valspeak

used to help anorexics toss up.

Yeah, Valley Girl talk. It was popular for a while in late 70's and early 80's, but, HEY, that's better than what some kids started saying shortly after that, "Gag a Maggot in a Gut Wagon"! Like, Totally!