After market gauges have different senders. For instance, I have VDO gauges matched with a 250* sender. Since I'm wiring my car now, I decided that I wanted a transmission temp sender as well (thanks, Henry Rifle - Jack). I looked through the VDO catalog and chose one identical to the engine coolant temp sender, not the 200* or 300* version. It's installed in my aluminum trans. pan with a SPDT switch so I can read one or the other.

Soooooo with that said, the gauge and sender need to match. Not the GM to gauge