GitLab does not support NUnitXml as provided by Pester. Instead, it supports JUnitXml. This helper function takes the Pester output and generates a JUnitXml from it to be consumed by GitLab pipelines

Convert-PesterResultToJUnitXml.ps1

function Convert-PesterResultToJUnitXml {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory)]
        $PesterResult
    )
    $junit = '<?xml version="1.0" encoding="utf-8"?>'
 
    $junit += "`n" + '<testsuites name="Pester" tests="{0}" failures="{1}" disabled="{2}" time="{3}">' -f @(
        $PesterResult.TotalCount,
        $PesterResult.FailedCount,
        ($PesterResult.SkippedCount + $PesterResult.PendingCount + $PesterResult.InconclusiveCount),
        ($PesterResult.Time.TotalSeconds.ToString('0.000', [System.Globalization.CultureInfo]::InvariantCulture))
    )
 
    $PesterResult.TestResult | Group-Object -Property Describe | ForEach-Object -Process {
        $results = $_.Group | Group-Object -Property Result
        $totalSeconds = ($_.Group | ForEach-Object -Process {
                $_.Time.TotalSeconds
            } | Measure-Object -Sum).Sum
 
        $junit += "`n" + '  <testsuite name="{0}" tests="{1}" failures="{2}" disabled="{3}" time="{4}">' -f @(
            $_.Name,
            $_.Count,
            ($results.Where{ $_.Name -eq 'Failed' }.Count),
            ($results.Where{ $_.Name -in 'Inconclusive', 'Pending', 'Skipped' }.Count),
            ($totalSeconds.ToString('0.000', [System.Globalization.CultureInfo]::InvariantCulture))
        )
 
        $junit += foreach ($testCase in $_.Group) {
            "`n"
 
            switch ($testCase.Result) {
                Passed {
                    '    <testcase name="{0}" status="Success" time="{1}" classname="" >' -f @(
                        $testCase.Name,
                        ($testCase.Time.TotalSeconds.ToString('0.000', [System.Globalization.CultureInfo]::InvariantCulture))
                    )
                }
                Failed {
                    '    <testcase name="{0}" status="Failure" time="{1}" classname="">' -f @(
                        $testCase.Name,
                        ($testCase.Time.TotalSeconds.ToString('0.000', [System.Globalization.CultureInfo]::InvariantCulture))
                    )
                    "`n"
                    '      <failure message="{0}" />' -f $testCase.FailureMessage
                }
                default {
                    '    <testcase name="{0}" status="Ignored" time="{1}" classname="">' -f @(
                        $testCase.Name,
                        ($testCase.Time.TotalSeconds.ToString('0.000', [System.Globalization.CultureInfo]::InvariantCulture))
                    )
                    "`n"
                    '      <skipped message="{0}" />' -f $testCase.FailureMessage
                }
            }
            "`n"
            '    </testcase>'
        }
 
        $junit += "`n" + '  </testsuite>'
    }
 
    $junit += "`n" + '</testsuites>'
    $junit
}

$res = Invoke-Pester -PassThru
Convert-PesterResultToJUnitXml -PesterResult $res |
    Out-File -FilePath ./gitlab-testresult.xml -Encoding utf8