Certainly "gullible" is a good English word, nearly 200 years old, with
several citations in the Oxvord English Dictionary. It was sometimes spelled
"gullable".

"Gullible" derived from the verb "to gull someone" meaning to cheat them,
attested from Elizabethan times. I think only "gullible" survives in English
now.

Two marvelous OED citations:

1825, Thomas Carlyle, in an essay about the 18th century German poet Friedrich
Schiller: "The king of quacks, the renowned Cagliostro, harrowing up the souls
of the curious and gullible of all ranks, by various thaumaturgic [wonder-
working] feats"

and

1822 Thomas Jefferson "With such persons gullability, which they call faith,
takes the helm from the hand of reason"

Bob Richmond
Samurai Pathologist and occasional lexicographer
Knoxville TN