The GM oil pressure gauges use a special type of compression fitting where the ferrule is built into the nut.
Weatherhead calls them "threaded sleeve" fittings and their 6100x2 nut will connect 1/8" tubing to the back of the stock gauge.
Edelmann calls them "double compression" fittings and their part number for the nut is 111200. The Everco equivalent is 11A.
I don't think anyone makes an adapter to go from the port on the stock gauge directly to AN.