'94-'01 LS, GS fits on '00-'01 GS-R, which differs from '94-'99 GS-R exhaust.
you can also get an exhaust made for an ITR, and it'll bolt right up...I think that's what the guy may have meant when he told you that.
if you want to check for yourself, go to Comptech's website and check the part #s for the Catback and headers for each of these cars, it's the same part.

I swear we need an F.A.Q., and this question needs to be at the top. I can't believe how many times it's been answered.