Hmmm..... I'm not so sure if you can use a pressure galley plug with that kit. Hopefully someone else can chime in, but I would suggest selling that gauge and buy one that has the galley plug pressure sender in the kit.
Edit: Did you buy the pressure plate separately?
If so, your kit should have pretty much everything you need for the install. You don't need to buy another oil pressure galley plug, your car already has one. You just unhook the stock sensor and screw the new pressure sending unit in. To create a solid seal, you can wrap the threads in Teflon tape before screwing it in.