Should p60 have gross or net pay on the form?

The P60 shows your taxable income and deductions and the information comes from the payer of the amounts to you.

Certificate by Employer/Paying Office:

This form shows your total pay for Income Tax purposes in this employment for the year.

Any overtime, bonus, commission etc, Statutory Sick Pay, Statutory Maternity Pay, Statutory Paternity Pay or Statutory Adoption Pay is included.