The linker cannot find this function definition,
double calcGross();
because you define the function as such,
double calcGross(int hoursWorked, double payRate)
Should you remove the parameters so that the function declaration & definition are matched. Change your function declaration like this,
int getHoursWorked();
double getPayRate();
double calcGross(int hoursWorked, double payRate);
Then pass the value of hoursWorked
& payRate
when calling it in main()