If i am taking penicillin for an infection and my child got gastroenteritis, am i immuned because im on antibiotics?

Not necessarily: It all depends on the type of virus or bacteria that is causing gastroenteritis to your son. Best protection remains hands hygiene.

Not from penicillin: Most childhood gastro is due to one of many enteroviruses. You are probably immune because you had the same strain in your past, although you may be susceptible if you didn't. The penicillin is unlikely to have any benefit beyond the treatment for whatever issue you take it.It does not grant you special defenses beyond certain things like strep throat.