2000 Chevy Silverado Temperature gauge not reading at normal operating temp?

Okay so last week my wife wasn't getting heat in her truck so i figured has to be the thermostat, so i changed that. Yesterday she's not getting any heat so I go to the parts store and pick up coolant flush and 2 gallons of coolant. I drain the coolant and put the flush in and top it up with water 20 minutes later It's still not reading on the temp gauge around 180. Anyone have any suggestions on what it could be? It is blowing out warm/hot air but it makes me weary when it's not reading normal on the temp gauge.