You can treat the results as arrays, which means you can add them together. This produces a result, but you'd have to verify it's the correct result.
21 ~> ((gp HKCU:\Software\Microsoft\Windows\CurrentVersion\Uninstall\*) + (gp HKLM:\Software\Microsoft\Windows\CurrentVe rsion\Uninstall\*) + (gp HKLM:\Software\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall\*)) | Select DisplayName, DisplayVersion, Publisher, InstallDate, HelpLink, UninstallString | ogv
On my machine I get several blank entries, making me wonder if this is working correctly. If it is, I'd add a where filter to clean the results up.