Candy is an old swamper. He fears losing his job because of his age. Similar to his owner, Candy's dog is also old and relatively useless. Readers will sympathize with Candy when he finally agrees to let Carson put the dog out of his misery. We might also sympathize with Candy because, in his advanced age, he is trying to seem useful enough to keep his job. He latches on to George and Lennie upon hearing of their dream because he sees this as a viable alternative to staying on the ranch.
Candy certainly is a gossip. Before we really learn anything about his age and his dog, we learn that he is a gossip and an eavesdropper. After George and Lennie meet the boss, George catches Candy listening to their conversation. Candy claims he wasn't listening but this is a lie. Candy gives George and Lennie the gossip on Curley and then pleads with them not to tell Curley he has said anything.
Candy eavesdrops and spreads gossip because he is lonely. He is trying to make friends. He is old and doesn't have a lot of leverage. In other words, he doesn't feel that he has any power because he feels he is not needed. So, to feel needed and useful, he spreads gossip. This at least gives him a chance to interact with others and to feel like he has something useful to offer: gossip, inside information, etc.