I think it works like this: that temp sensor tells the ECU, the ECU tells the temp gage and the fan what to do. So if this is different than before the gasket project there is something different about the wiring harness or fan or computer or temp sensor than before. Beyond that, a wiring diagram, multimeter and a troubleshooting plan are what is needed to find and fix. Most dealers are not equipped materially or mentally to handle this so you are IT!
Karl