The way it was normally done (back in the 70s) for an after market fitting is to find the sender for the oil pressure warning light, remove it from the block, or wherever, and fit a "T" piece, so the sender for the gauge and the warning light still function.
Some modern cars (2009 Roadster, for example) have both gauge and warning light from one sender, but I've no idea if one could substitute a combination sender. I suspect the sender and guage are "matched" electrically.
The problem with modern engines, such as the GDI, is that most of the data from senders goes into the ECU first. So tampering might cause problems, so the 1970s method might be the only way.
Good luck!
