Its the syncormesh gears in the gearbox wearing out, its a common "problem" in Nissan performance cars. Get on down to Rare Spares in Phillip and get some Redline Lightweight Shockproof oil (tell them its for a R33 GTST and they should be able to sell you the correct amount). Change the oil in the gearbox yourself (im sure theres a guide in the DIY section) or take it to a mechanic to change. This will usually fix up the notchy gear changing issue in these gearboxes
As for servicing, you can strecth it out to 10,000kms if the car is stock, but if you plan on keeping it for a while spend the extra and do it every 5000kms. Oil is critical to these cars, and if you start scrimping on it things will start going wrong . Id personally reccomend at least a semi synthetic such as Motul 4100 Turbolight, or if you have money to burn, Motul 300V Chrono. Once again, these oils can be picked up from Rare Spares in Phillip
Other than that, just enjoy the car! Oh we also meet up at the National Museum on Friday nights from about 9pm, you should swing past