Either gull + -ible, or from dialect Middle English gull (“newly hatched bird”), perhaps from Old Norse gulr, from the hue of its down.
gullible (comparative more gullible, superlative most gullible)
- Easily deceived or duped; naïve, easily cheated or fooled.
Derived terms 
easily deceived or duped, naïve
- ^ “gullible” in Douglas Harper, Online Etymology Dictionary (2001).