Haven't got long so I will be brief (please don't mistake this for rudeness).

First, you have a typo ($DirveSize instead of $DriveSize):

$DriveSizeFree = $DirveSize.FreeSpace / 1GB

Secondly, $DriveSize is an array of objects with Name, Size and FreeSpace properties. So $DriveSize.FreeSpace makes no sense but $DriveSize[0].FreeSpace does (assuming there is at least one member in the array).

Hope this gets you further!!!