Garfield regards Jon's accusations that he is disgusting as merely a statement of fact, rather than a complaint or criticism, as indicated by the cat's retort which adds... well, insult to insult. That Jon's observation has no effect is hardly surprising, since the "prank" that is belching in someone's face is enacted because it is disgusting. Garfield counters with not just another fact, but by 1) communicating that the horrible thing Jon has just experienced has the further consequence of depleting household supplies, (2 implying that Jon should now feel obligated to replenish the soda, which in turn (3 sets up the circumstances for Garfield to blow stomach air in Jon's face again.
This strip is about abuse cycles. Except, of course, that Garfield will never demonstrate remorse, so when Jon inevitably enters the one-man-honeymoon stage and buys more soda, he is not just manifesting his own self-esteem issues, but accepting his role in Garfield's Theater of Cruelty.