The later model RB25 and series II have different length valve springs on the intake and exhaust side, so you can not use GTR springs unless the spring seat in modified to suit.
Both GTR and RB30 are top flow, but Rb30 are a barbed fitting that requires fuel hose where as GTR are oringed and slip into the fuel rail.
GTR could be modified to suit RB30 though with alot of hassel.'
Cheers