First, welcome to the site. Good to see you here.

Next, you can do a simple test to verify if the gauge failed or the sending unit. Simply touch the sending unit wire to ground and see if the gauge changes state. It should go to its max reading. If it does, then indeed it is the sending unit. I think most of the after market use the same sending unit. I took out my TPI gauges and replaced with Autometer, the sending unit wasn't replaced on the temp or oil gauges, didn't make a difference.