Has since been inverted, subverted, parodied, deconstructed, averted, played for laughs, lampshaded, defied, discussed, and exploited. Needless to say, it's seen very extensive use.
Overused Running Gag: The voluminous number of "Garfield (pun for Minus) Garfield" comics has now reached this, to the point of Seasonal Rot for some.
Hopefully as of strip 625 we should be seeing far less of them now.
Slow Loading Internet Image: While there's no punchline, and it's a non-pornographic image, the spirit is certainly there in this strip.
Something Completely Different: It was more common in the early days of the strip, but there are a few strips that are not Garfield edits: Edits of other comics (xkcdhere, Krazy Kathere, Bob and Georgehere, and Hark! A Vagranthere.), unedited pages about Garfield from comic books or magazines (this from a Ren and Stimpy comic book, and this from a The Electric Company magazine), and, on one occasion, a printing error a newspaper made when publishing a Garfield comic. Possibly lampshaded in the author's notes for the Krazy Kat strip, which said about the strip "This one may be too meta to fly.".
Your Television Hates You: This strip, where Garfield watches TV and every show on is something that has been referenced at least once in an earlier SRoMG comic.