Good morning,
Please use this method when posting code to the forum as it makes it easier for the people working with it.
Guide to Posting Code - Redux - PowerShell Help - PowerShell Forums
Here is an article I found on your topic
Convert bytes to gigabytes with PowerShell – 4sysops
I would recommend using Get-Ciminstance as the Get-WMIObject is deprecated.
I put this together, it should give you the expected results.
Get-Ciminstance win32_diskpartition | Select Name,Index,
@{name="SizeGB";e={[math]::round($_.size /1GB)}},
@{name="OffSetGB";e={[math]::round($_.startingoffset /1GB)}}