Can I change the gross pay of the employee every month? How will it affect tax calculations?

Hello accounts-charity,

It's a pleasure to see you in Community. Welcome! It's essential that your payroll transactions are recorded accurately in QuickBooks Online, to ensure they're aligned with government guidelines.  For this reason, I recommend  speaking with an accounting professional for their expertise and best course of action. If you don't have an accountant, we can help you locate a ProAdvisor in your local area. 

If you have any other questions, feel free to reach back out. We would be glad to help!