Offline GitLab Runners are not automatically unregistered, nor is there an easy GUI way to remove them. This script uses GitLab API to remove the runners.

GitLab-Runner-Offline-Unregister.ps1

#######################################
# Please replace this values
$gitlabUrl = "https://gitlab.com"
$bearerToken = "abcdefg"
#######################################

$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("Authorization", "Bearer $bearerToken")
$runners = Invoke-RestMethod "$gitlabUrl/api/v4/runners?status=offline" `
            -Headers $headers

if ($runners) {
    $runners
    Pause
} else {
    Write-Host "There were no offline runners"
    Exit
}

foreach ($runner in $runners) {
        $id = $runner.id
        Invoke-RestMethod "$gitlabUrl/api/v4/runners/$id" `
                -Method DELETE -Headers $headers
}