The ScriptText parameter accepts a string that contains the code to be executed inside the guest OS.
The cmdlet is not going to read the content of your file.
But you can do
$script = 'c:\users\gman\desktop\insertPS.ps1'
$code = Get-Content $script
Invoke-VMScript -ScriptText $code -VM $VMs -GuestUser administrator -GuestPassword 'jjw1ldT$G'