How to do looping for checking an existing folder with PowerShell?

Ok, first of all, you are using different tests for the file and/or directory in the code and in the GUI. Furthermore, you call the GUI.ps1 file with a path set to a $null value.

I would change your code to something like this:

$Path      = "D:\Process\*\SSID_LST\*"  # the path to look for files
$SSID_Unit = "111dddddfafafesa"         # the Search pattern to look for inside the files

function Test-FileWithGui {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory = $true, Position = 0)]
        [string]$Path,
        [Parameter(Mandatory = $true, Position = 2)]
        [string]$Pattern,
        [int]$MaxAttempts = 5
    )

    Write-Host "Starting Mapping SSID and Finding Job"

    # set up an 'empty' $global:Result object to return on failure
    $global:Result = '' | Select-Object @{Name = 'Exists'; Expression = {$false}}, FileName, Directory, @{Name = 'Attempts'; Expression = {1}}

    # test if the given path is valid. If not, exit the function
    if (!(Test-Path -Path $Path -PathType Container)) {
        Write-Warning "Path '$Path' does not exist."
        return
    }
    # try and find the first file that contains your search pattern
    $file = Select-String -Path $Path -Pattern $Pattern -SimpleMatch -ErrorAction SilentlyContinue | Select-Object -First 1

    if ($file) {
        $file = Get-Item -Path $file.Path

        $global:Result = [PSCustomObject]@{
            Exists    = $true
            FileName  = $file.FullName
            Directory = $file.DirectoryName
            Attempts  = 1
        }
    }
    else {
        & "D:\GUI.ps1" -Path $Path -Pattern $Pattern -MaxAttempts $MaxAttempts
    }
}

# call the function that can call the GUI.ps1 script
Test-FileWithGui -Path $Path -Pattern $SSID_Unit -MaxAttempts 20

# show the $global:Result object with all properties
$global:Result | Format-List

# check the Global result object
if ($global:Result.Exists) {
    Write-Host "File '$($global:Result.FileName)' Exists. Found after $($global:Result.Attempts) attempts." -ForegroundColor Green
}
else {
    Write-Host "File not found after $($global:Result.Attempts) attempts." -ForegroundColor Red
}

Next the GUI file. As you are now searching for a file that contains some text, you need a third parameter to call this named Pattern.

Inside the GUI file we perform the exact same test as we did in the code above, using the parameter $Pattern as search string:

Param (   
    [string]$Path,
    [string]$Pattern,
    [int]$MaxAttempts = 5
) 

Add-Type -AssemblyName System.Windows.Forms
[System.Windows.Forms.Application]::EnableVisualStyles()

# set things up for the timer
$script:nAttempts = 0
$timer = New-Object System.Windows.Forms.Timer
$timer.Interval = 1000  # 1 second
$timer.Add_Tick({
    $global:Result = $null
    $script:nAttempts++

    # use the same test as you did outside of the GUI
    # try and find the first file that contains your search pattern
    $file = Select-String -Path $Path -Pattern $Pattern -SimpleMatch -ErrorAction SilentlyContinue | Select-Object -First 1

    if ($file) {
        $file = Get-Item -Path $file.Path

        $global:Result = [PSCustomObject]@{
            Exists    = $true
            FileName  = $file.FullName
            Directory = $file.DirectoryName
            Attempts  = $script:nAttempts
        }
        $timer.Dispose()
        $Form.Close()
    }
    elseif ($script:nAttempts -ge $MaxAttempts) {
        $global:Result = [PSCustomObject]@{
            Exists    = $false
            FileName  = $null
            Directory = $null
            Attempts  = $script:nAttempts
        }
        $script:nAttempts = 0
        $timer.Dispose()
        $Form.Close()
    }
})

$Form             = New-Object system.Windows.Forms.Form
$Form.ClientSize  = '617,418'
$Form.Text        = "AutoGM"
$Form.BackColor   = "#8b572a"
$Form.TopMost     = $true
$Form.WindowState = 'Maximized'

# I have removed $Label2 because it is easier to use 
# just one label here and Dock it to Fill.
$Label1           = New-Object system.Windows.Forms.Label
$Label1.Text      = "UNDER AUTOMATION PROCESS`r`n`r`nWaiting for the job..."
$Label1.AutoSize  = $false
$Label1.Dock      = 'Fill'
$Label1.TextAlign = "MiddleCenter"
$Label1.ForeColor = "#ffffff"

$L_S = (($Form.Width/2) - ($Form.Height / 2)) / 10
$Label1.Font = "Microsoft Sans Serif, $L_S, style=Bold"

$Form.controls.Add($Label1)

# start the timer as soon as the dialog is visible
$Form.Add_Shown({ $timer.Start() })

[void]$Form.ShowDialog()

# clean up when done
$Form.Dispose()

The results during testing came out like below

If the file was found within the set MaxAttempts tries:

Starting Mapping SSID and Finding Job


Exists    : True
FileName  : D:\Process\test\SSID_LST\blah.txt
Directory : D:\Process\test\SSID_LST
Attempts  : 7



File 'D:\Process\test\SSID_LST\blah.txt' Exists. Found after 7 attempts.

When the file was NOT found:

Starting Mapping SSID and Finding Job


Exists    : False
FileName  : 
Directory : 
Attempts  : 20



File not found after 20 attempts.

If even the folder $Path was not found, the output is

Starting Mapping SSID and Finding Job
WARNING: Path 'D:\Process\*\SSID_LST\*' does not exist.


Exists    : False
FileName  : 
Directory : 
Attempts  : 1



File not found after 1 attempts.

Hope that helps