Gabby describes a person who talks a lot, often about small things, and does not stop easily. It is an adjective, not a verb. A gabby neighbour tells you the whole story when you only asked the time. It can be fond — you like the person, you just notice the river of words — or a little critical, as if they use too many words and leave too little air. It is not as strong as rude names for someone who never lets you speak. You use it about people, and sometimes about a chatty style, not about a serious speech or a careful book. It is informal: fine with friends, odd in a formal essay. It is built in a simple way: gab plus -y, the same kind of ending as noisy or messy.