I made this PowerShell script for you
Add-PSSnapin Microsoft.SharePoint.PowerShell -EA SilentlyContinue
$listName = "CustomList1"
#Get the SPWeb object
$web = Get-SPWeb http://spwapp/
#Get the SPList
$list = $web.Lists[$listName]
#Get all items in this list
$items = $list.items
#Go through all items
foreach($item in $items)
$created = $item["Created By"]
$item["DestinationPG"] = $created
#Update the item
Copy and Paste this script, refer to your SP2013 site url and your List name, and execute it, it will update all the items at once.
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