What tools would you recommend for comparing two GPO and identifying differences? AGPM is not the option as we do not have a Software Assurance contract with Microsoft.
Is there anything else?
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I tend to use my eyes, actually. Open two instances of Group Policy manager, put one on one screen, one on the other screen and go through it.
You could also possibly use a text or code diffing tool if you opened up the objects in a text editor... but that really seems like a lot more work than putting the two GPOs up, side-by-side and looking at them.
How about exporting the GPOs you want to compare in a file and then use any tool you want to compare the contents.
You can export the GPOs using Powershell
You can find more info about using Powershell GPO cmdlets here