I have installed an oil pressure gauge, a voltmeter, and a temperature gauge. You can't install a "T" fitting for the oil pressure gauge because the 2.4L doesn't have a sending unit. It just has a switch that indicates that there is , or is not, oil pressure. You will need to install a sending unit unless the GXP has one. I believe I saw a post that indicatied that the oil pressure sending unit or pressure switch on the GXP is very difficult to get to and that it is easier to install a sending unit in the same place I did. The following post details how I did my gauge installation.
Unique Custom Gauge Pod
I found the installation to be quite easy. Let me know if I can provide you with any more information.
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2008 Sly 2.4L – Premium Package, Preferred Package, Convenience Package, A/C, Monsoon, Chrome wheels
Mods: GM catback exhaust, CAI, de-badged, custom door handle backings, black painted calipers, splash guards, covered visor warning labels, seat back swap, waterfall organizer, custom gauge pod with Dakota Digital oil pressure, voltmeter and temperature gauges, Vector tune, shortened antenna, padded, leather covered center console, JPM shifter, custom knob and leather boot.