How can I list only groups with a gidNumber?

I need to export a list of ad groups and their gidNumber. This is what I have so far. My issue is that this also gives me groups without gidNumbers and I don't want those. How can I filter out groups that don't have a gidNumber?

Get-ADGroup -Filter "*" -Property gidNUmber | select Name, gidNumber | Sort-Object -Property gidNumber | export-csv -Path C:\Labs\exportgroups.csv -NoTypeInformation