All men should live in mortal fear of their pets. The terror on Jon's haggard face, and hand raised in defensive "don't-hit-me" position are my idea of a truly fine punchline.
Garfield thinks gurgling phlegm sounds like a meal announcement, which is gross. Why hunger (or gluttony) inspires Garfield to smell Jon's eyeball, I don't know. It's intimidating when somebody's all up in your grill like that, but it must be nightmarish if they are smiling and bug-eyed.
The first two panels are amazing: imagine a civilization who has never encountered Garfield. A race of space bug-men, early America peoples, or whatever. If you don't intimately understand the shorthand of comics in general and the iconography of Garfield in specific, panels one and two are just abstract shapes. There's no way panel one would read as a cat sleeping in a box with a blanket pulled over his face. No way does panel two look anything like a cat waking up and running from beneath a blanket.
Panel two looks like something is ramming into a suspiciously vaginal cave. Then again, if I didn't know better, I'd say panel three shows a cat with popcorn falling out of his butt.