a/f gauge

Assuming its a narrow band [0-1V] three wire A/F gauge:
RED wire to 'ACC' switched +12V power source;
BLACK to (-) ground;
SIGNAL wire could splice into the wire going to the GTIR ECU's pinout #19 ,
or perhaps run the wire directly to the O2 sensor signal connection at its location on the exhaust system.
(ECU Pinout Diagram seen in 'most viewed articles' header above).
If there is a fourth wire on the brand of gauge you are using it could be for illumination and just runs to any ACC switched power source.

if your going to the trouble to install a narrow band you might as well spend some money on a wide band,,, other wise you have a usless disco light in your car... just my opinion