Use existing .NET classes instantly with PowerShell by using [class]::Method(args):

PS C:\\> [guid]::NewGuid()

Guid
----
8874a185-64be-43ed-a64c-d2fe4b6e31bc

Similarly, in PowerShell 5+ you may use the New-Guid cmdlet:

PS C:\\> New-Guid

Guid
----
8874a185-64be-43ed-a64c-d2fe4b6e31bc

To get the GUID as a [String] only, referenced the .Guid property:

[guid]::NewGuid().Guid