Public/Users/Get-GSUserAlias.ps1

function Get-GSUserAlias {
    <#
    .SYNOPSIS
    Gets the specified G SUite User's aliases
    
    .DESCRIPTION
    Gets the specified G SUite User's aliases
    
    .PARAMETER User
    The primary email or UserID of the user who you are trying to get aliases for. You can exclude the '@domain.com' to insert the Domain in the config or use the special 'me' to indicate the AdminEmail in the config.

    Defaults to the AdminEmail in the config
    
    .EXAMPLE
    Get-GSUserAlias

    Gets the list of aliases for the AdminEmail user
    #>

    [cmdletbinding()]
    Param
    (
        [parameter(Mandatory = $false,Position = 0,ValueFromPipelineByPropertyName = $true)]
        [Alias("PrimaryEmail","UserKey","Mail","Email")]
        [ValidateNotNullOrEmpty()]
        [String[]]
        $User = $Script:PSGSuite.AdminEmail
    )
    Begin {
        $serviceParams = @{
            Scope       = 'https://www.googleapis.com/auth/admin.directory.user.readonly'
            ServiceType = 'Google.Apis.Admin.Directory.directory_v1.DirectoryService'
        }
        $service = New-GoogleService @serviceParams
    }
    Process {
        foreach ($U in $User) {
            try {
                if ($U -ceq 'me') {
                    $U = $Script:PSGSuite.AdminEmail
                }
                elseif ($U -notlike "*@*.*") {
                    $U = "$($U)@$($Script:PSGSuite.Domain)"
                }
                Write-Verbose "Getting Alias list for User '$U'"
                $request = $service.Users.Aliases.List($U)
                $request.Execute() | Select-Object -ExpandProperty AliasesValue
            }
            catch {
                if ($ErrorActionPreference -eq 'Stop') {
                    $PSCmdlet.ThrowTerminatingError($_)
                }
                else {
                    Write-Error $_
                }
            }
        }
    }
}