I dont think you'll find any guids for software not installed by Windows Installer.
https://learn.microsoft.com/en-us/windows/win32/msi/product-codes
the thread linked below has a good rundown of different places to check. first thing that came to mind to me was to use get-ciminstance. Careful tho, msi may attempt to self repair if it thinks there's a problem with the software.
get-ciminstance Win32_Product | Sort-Object -Property Name |Format-Table IdentifyingNumber, Name, LocalPackage -AutoSize