the 2530 is a good little turbo that will improve performance everywhere. exactly like tangles says. make sure it is a T3 housing if you buy one.
the GCG unit will get you more top end so for me that is what i would pick especially knowing it is new to begin with vs 2nd hand (caveat emptor) off ebay. i have a HKS GT-RS on the stagea but the price due to currency has gone a bit too high compared to the GCG unit at this point (they are pretty line ball performance wise).
a nismo FPR can push your injectors a bit which will do the trick with either turbo. the FPR is not model specific so it won't matter which RB you have.
as a comparison i have a GT3040 on the R33 with a HKS manifold and external gate all parts extrude honed etc etc (ex-legend01). buying the gear was cheap enough but fitting it all.....
new injectors, new clutch, exhaust mods, engine mount mods, intake mods, more tuning, more exhaust mods due to noise increases (all workshop stuff) then custom fabricated airbox, custom fabricated manifold heat shield (ok the custom bit means me pretending in the garage with sheet aluminium), moving carbon canister to make room for oil catch can, running around finding pipes, clamps hoses to plumb it all in.
ultimately the response of the stagea is much stronger at lower rpm.
the R33 does haul from 3500rpm and that is more than ok too but i wouldn't like this turbo on the stagea.
ps. the GT3040 has either a too small turbine or too large compressor (depending on glass half full or glass empty) to be a really good thing so this is a compromised turbo but this combo works really well even on the RB20 it came off.