there is no set amount of power you will gain, but over the gsr cams you should notice a nice gain in power. If you do not change the stock rev limit you do not need to do any other modifications. If you want to rev higher you will need to upgrade your valvesprings. After '00 ITR and CTR cams are the same.
if i am not mistaken you just need to upgrade your intake valve springs to ITRs and move your GSR intake valve springs over to the exhaust side, now is also a good time to upgrade to tit retainers, if you desire to do so.
i'm running CTR cams on stock valvetrain and ecu in my del sol, works great. you only really need to upgrade your valvetrain if you plan on revving past stock rev limit, otherwise it is not all that necessary.