Credulous, gullible both mean unduly trusting or confiding but they differ significantly in their implications as do their corresponding nouns credulity and gullibility.
Credulous and credulity stress a tendency to believe readily and uncritically whatever is proposed for belief without examination or investigation; typically they suggest inexperience, naïveté, or careless habits of thought rather than inherent incapacity.
Gullible and gullibility, on the other hand, stress the idea of being duped; they suggest more the lack of necessary intelligence than the lack of skepticism, and connote the capacity for being made a fool of.