[Stroker Ace Ventura]

Go inside and boil a pan of water, bring it out and stick the probe in it... That will at the very least confirm whether it's reading engine coolant temp or something else is wrong. Is this ATM 3337 a capillary temp gauge or is it electronic? Looked it up and no pics of how to install or what the sending unit looks like.. I looked everywhere.

 

[Stroker Ace Ventura]

Thats the model :

Auto Meter 3337 Sport-Compact Short Sweep Electric Water Temperature Gauge, Water Temp - Amazon Canada

Auto Meter 3337 Sport-Compact Short Sweep Electric Water Temperature Gauge in Water Temp.

www.amazon.ca

is there another way to be sure the gauge itself is bad because everything is new? By bench test all this together?

You could totally bench test it, just need a 12VDC power source and hot water with a thermometer... Should be pretty simple.
Take the gauge out (or leave it in) and just use a remote 12VDC power source and a pan of hot water... Bypassing the vehicle's wiring

 

[Stroker Ace Ventura]

Thinking the sending unit is roached. I had a similar issue with my temp gauge, I had killed it from a short. Never read past like 0° F anything!