Synonyms

Definition of stupid:

adjective: stupid; comparative adjective: stupider; superlative adjective: stupidest

1.

having or showing a great lack of intelligence or common sense.

"I was stupid enough to think she was perfect"

synonyms:unintelligent, ignorant, dense, foolish, dull-witted, slow, simpleminded, vacuous, vapid, idiotic, imbecilic, imbecile, obtuse, doltish; 

informalthick, dim, dimwitted, slow-witted, dumb, dopey, dozy, moronic, cretinous, pea-brained, halfwitted, soft in the head, brain-dead, boneheaded, thickheaded, wooden-headed, muttonheaded, daft

"they're rather stupid"

foolish, silly, unintelligent, idiotic, scatterbrained, nonsensical, senseless, harebrained, unthinking, ill-advised, ill-considered, unwise, injudicious;

inane, absurd, ludicrous, ridiculous, laughable, risible, fatuous, asinine, mad, insane, lunatic;

informalcrazy, dopey, cracked, half-baked, dimwitted, cockeyed, lamebrained, nutty, batty, cuckoo, loony, loopy

"that was a really stupid thing to do"

antonyms:intelligent, sensible


Definition of gullible:

adjective: gullible

easily persuaded to believe something; credulous.

"an attempt to persuade a gullible public to spend their money"

synonyms:credulous, naive, overtrusting, overtrustful, easily deceived, easily taken in, exploitable, dupable, impressionable, unsuspecting, unsuspicious, unwary, ingenuous, innocent, inexperienced, unworldly, green; More

informalwet behind the ears, born yesterday

"he was a swindler who preyed on gullible elderly widows"

antonyms:suspicious



In the Rifleman's opinion, they are synonymous.  One is simply less harsh than the other. The more harsh is also more widely encompassing. Not all stupid acts are acts of gullibility,  but all acts of gullibility are stupid acts.