I also tried various resistors in line with the sensor with either no additional movement or a total peg to the hot position. Don't think that is the way to go. I've got a couple of inquiries in to some NOS parts suppliers on a gauge and sender. Waiting for a response from them. By the way, the needle on the NOS Chevy gauge that I have also reacts the same way as you turn it around. Moves freely between 100 and 212 degrees. So your gauge is probably fine.
