Chatterbox, though he likes to piously claim that he only watches C-Span, can't resist passing along the latest anchor-amok TV gossip. Easy-going Charlie Gibson, recently liberated from dawn- patrol duty on Good Morning, America/, was such a hit last week filling in on the ABC evening broadcast that a certain Canadian-born news-reader has become a tad jumpy. So how paranoid is Peter Jennings, whose $7-million-a-year contract runs until 2000? Let's just say that the latest joke around ABC is that Gibson had better hire a food taster. With ABC now often running third in the news-rating wars battle, Jennings has by all accounts replaced the now-mellow Dan Rather as the anchor most likely to be mistaken for Captain Queeg.