the G3 GSR intake manifold has a two stage intake runner setup. it has long runners at low RPM which help with low range torque production and at 5800 opens up to the high RPM short runners to help high RPM breathing, which produces more top end HP. FYI: it's not worth changing the manifold on a GSR unless you have intake, header, exhaust, cams, etc. you will lose HP if you do it on a car with stock cams.
PS - what do you have and what are you looking to do? the more info you got the better info we can give