There was no universal uninstaller for the GroupWise client that functioned the way we needed, so this script will simply find the installation in the registry and run the uninstall via MSIEXEC.

Remove-GroupWiseClient.ps1

if (Test-Path HKLM:\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall) {
    $gwreg = Get-ChildItem HKLM:\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall | foreach { 
        Get-ItemProperty $_.pspath | where { 
            $_.DisplayName -like '*GroupWise*' 
        }
    }
}
else {
    $gwreg = Get-ChildItem HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall | foreach { 
        Get-ItemProperty $_.pspath | where { 
            $_.DisplayName -like '*GroupWise*' 
        }
    }
}

foreach ($gw in $gwreg) {
    Write-Verbose "Removing version $($gw.DisplayVersion) of $($gw.DisplayName)"
    Add-Content -Value "$(Get-Date)" -Path C:\LOGS\GWUninstall.log
    Add-Content -Value "Attempting to remove version $($gw.DisplayVersion) of $($gw.DisplayName)" -Path C:\LOGS\GWUninstall.log
    Add-Content -Value "`tMSI Key: $($gw.PSChildName)" -Path C:\LOGS\GWUninstall.log
    Add-Content -Value "`r`n" -Path C:\LOGS\GWUninstall.log
    $arguments = "/X$($gw.PSChildName) /qn /norestart /Limew `"C:\LOGS\$($gw.DisplayName).log`""
    $process = Start-Process -FilePath "C:\Windows\System32\msiexec.exe" -ArgumentList $arguments -Wait -PassThru
    if ($process.ExitCode -eq 1639) {
        Add-Content -Value "Error 1639 - ERROR_INVALID_COMMAND_LINE" -Path C:\LOGS\GWUninstall.log
        Add-Content -Value "Attempt #2 to remove version $($gw.DisplayVersion) of $($gw.DisplayName)" -Path C:\LOGS\GWUninstall.log
        Add-Content -Value "`r`n" -Path C:\LOGS\GWUninstall.log
        $arguments = "/X$($gw.PSChildName) /qn /norestart /Limew C:\LOGS\gw-pull.log"
        $process = Start-Process -FilePath "C:\Windows\System32\msiexec.exe" -ArgumentList $arguments -Wait -PassThru
    }
    if ($process.ExitCode -eq 0) {
        Add-Content -Value "SUCCESS - Uninstall completed... :)" -Path C:\LOGS\GWUninstall.log
    }
    else {
        Add-Content -Value "FAILED - Something went wrong! :(" -Path C:\LOGS\GWUninstall.log
    }
    Add-Content -Value "`r`n" -Path C:\LOGS\GWUninstall.log
}

# http://stackoverflow.com/questions/4124409/run-msiexec-from-powershell-and-get-return-code