When you gabble, you talk so fast that you can barely be understood. A nervous public speaker might gabble for several minutes before she's able to get her point across.
If you're gossiping about a neighbor and suddenly realize he's standing behind you, you might gabble for a while from the sheer awkwardness of the situation. Your grandmother might declare that she doesn't understand the music you like, saying, "They don't sing — they just gabble!" Gabble is a noun, too, meaning the sound itself: "See? It's all just gabble!" Gabble has a Dutch root, gabbelen, which is imitative — it sounds just like what it means.