Jesse isn't that stupid. He knows Gus had the gift with him from the very beginning.
Because Gus suspected the cartel would try to keep Jesse in Mexico, and had the gift as Plan B in case Jesse needed rescuing.
Or so Gus will let Jesse think.

Because that plot twist is hackneyed. Makes him look much more badass if he used some kind of poison that there was no guaranteed antidote for.I wonder why he didn't put in some kind of poison that he could have preemptively taken an antidote for?