The gross wage does not include the shift amount because the wage formula setting is incorrect. Please follow the steps below to change it:
Go to the Payroll module, Setup, Wage Formulae Setup.
Select Wage ID as Gross Wage.
Move the Shift Amount over.
Move over, All Deductions. (If you do not want the Total Gross Wage to include statutory deductions)
Move over All Allowance (if your allowances are not included in the gross wage)
Click Save.