They're so similar, The main difference would be that gullible mean easily fooled while credulous mean you believe things very quickly. "I'm not gullible enough to believe something that outrageous. They sell overpriced souvenirs to gullible tourists! " "Kara took the Joke seriously because she was too credulous to understand the sarcasm." "Credulous people can be easy targets tor scams.

I don’t think there is a difference in meaning. “Gullible” is a much more common word, and 100% of people know it. “Credulous” is less common, and some native speakers won’t know what it means. There may be a slight difference in tone: “Gullible” connotes a strong negative judgment (“Wow, you really are a fool!”), while “credulous” might be a softer, less critical way of saying pretty much the same thing.

They mean the same thing, but gullible is the more common term. They both describe someone who is naive, inexperienced, too trusting, and not "street smart."