The car has heat now but the temp. gauge only registers to about 180.
Normal is 192 to 195 as long as it has the stock T-stat in it.

when she start driving again she had no heat.
Explain "no heat"; the fan ran and was blowing cold air or the fan wouldn't run at all.

Also, which year and model Grand Prix? Manual A/C or Auto A/C (1998-2000)?