How to convert a $size variable from bytes into GB in Powershell?

there is a trick for it

$result.size = $size[$_] /1Gb

and if you want better view for results you can truncate it

$result.size = [math]::round($size[$_] /1Gb, 3)

http://technet.microsoft.com/en-us/library/ee692684.aspx