This is undoubtedly more of a vague exclamation than most sayings. "Gag me with a spoon," is not an idiom that reveals some truth about life or imparts any pearls of wisdom. Rather, it conveys a distinct disgust at something disturbing or in most cases, hypo-criticism.
It requires a certain elasticity of facial features to really say "Gag me with a spoon" correctly. Namely, the lip must curl ever so slightly, and one eyebrow must be raised in a hint of superiority.
And "spoon" must not be spoken with only one syllable, it must be given two to achieve the proper element of scorn. "Spoo-oon!" Or in some cases of extreme horror, the expression may be ended with "Gag me with a pitchfork." Please don't ask me why. However, you must be careful: like as not, the only person who can rightly use the expression this way is my dad. If you use the pitchfork ending, people will just look at you like you are weird.