There is no in between, either full or empty, Speedomter does not work either.
Gas gauge either showes "full" if there is any gas, Showes empty when "empty". on 1999 Buick Century
The fuel tank sender unit has most likely failed it is in the fuel tank and takes about 1.5 hours to change. Does the cruise control work? If cruise control works problem may be in instrument cluster if it doesn't the vehicle speed sensor on the transmission may have failed, about .7 hour to replace plus price of part and testing.