You should have bought an oil pressure sender with your guage. usually about 25 quid from rally design, deamon tweaks whatever. There are cheaper alternatives, like a standard vauxhall sender(as fitted to astra gte), but they only read up to 5 BAR and i expect your guage goes up to 8.
You can either buy a T piece that plugs into where your standard low pressure switch is fitted (rear of oil pump), or you can utilise another spare blanking plug near the oil pump. The thread of the standard low pressure switch is M14.
Once you have a sender plumbed in its simply just a single signal wire from the sender to the guage, then a switched positive and earth on the back of the guage, possibly power for illumination if you have it.