The total Gross amount is based on all the CPF payable pay elements on the system.
Here’s how you can check if the pay elements of your staff are CPF payable:
Step 1: Go to 
Step 2: Click on 
Step 3: Look for the name of the staff and click on the 
Step 4: In each of the pay elements that can be seen on the staffs' pay information, look if the CPF box is ticked:
Since the CPF box is ticked, once you process the payrun of this staff, it will show the Gross amount:
If you will process the payrun of your foreign staff and the CPF is unticked for all his pay elements, then the Gross amount there will show 0:
Please note that Foreign staff will not be deducted by CPF even if you ticked the CPF payable box of their pay elements.