Lookup Row in Google Sheets and either create new row (if entry doesn't exist) or update existing row with new data.

HI there, I am having difficulty setting up my desired workflow, or at least understanding how to get Zapier to do what I want.

The desired workflow is: When a form is submitted on Gravity forms on our website (i.e when a reccouring donation is set up) I want Zapier to check a google sheet to see if that person has already done this in the past. If not, then it should add a new row to the bottom of the sheet with their details, including monthly donation amount. If they HAVE done this process before (often happens if someone chooses to raise or lower their monthly amount) - then I want Zapier to find the person and update the “donation amount” column for them with the amount submitted in the Gravity Form.

So far, I am confident in what I have set up under the “Find or Create Row in Google Sheets” action as the initial step after bringing in the data from Gravity Forms.

Next I have added an action of “Update Spreadsheet Row in Google Sheets” but have got stuck on trying to work out what I should put in the “Row” box.

Under it the description reads: “If you want to update the same row each time select a row from the dropdown box. To dynamically update the row, click open the dropdown and choose the "Custom" tab, then the "Add a search step" button.”

So, I do want to dynamically update the row, but am A) wondering if I really need to add a search step, can I not just use the existing “lookup” step that is already there; and B) what is the “value” that I should choose? Do I choose the “first name” row? That should then search for any row that has the submitted name from gravity forms, right? But what if there are two people with the same first name or last name? Would it work if I add two columns of data into the “row box”? Or do I need to merge them into one “full name” box?

I hppe this is all clear enough,
Thanks in advance for any and all support.

Kind regards,
Jake