is workshop a relevant experience to be considered for a graduate school admission? i know it's not as important as GPA but does it have any noticable effect to be admitted?
of course i am talking about workshop that is related to the graduate school.
Most workshops would likely have little if any effect, other than to show your interest in the field. 

There could be exceptions, of course. Long workshops (months), perhaps. Workshops in which you build something significant, perhaps. Workshops that result in letters of recommendation that highlight something especially interesting, perhaps. 

You can mention such things in your CV, but don't expect a lot of bang.