gullible (adjective) – naïve, believes things easily
gullibility (noun) – naiveté
gullibly (adverb) – naively
Here are some examples of how you can use it:
Gullible tourists thought they were buying thousands-year-old antiquities, but in fact they were mass-produced at a factory.
Aren’t you ashamed taking advantage of a young gullible teenager?
I was young and gullible, so I gave him my money.
Sports judges aren’t known for their gullibility, yet they somehow failed to notice the widespread drug use amongst cyclist for years and years.
Do you really think your teacher will gullibly forgive you if you tell him your dog ate your homework?