Apple mdls (MetaData List) command on Networked Drive
Just an FYI, exiftool (which has to be installed) can pull all metadata from the file over the network without the need to have the network volume indexed.

That said, the most likely reason you're only getting some of the metadata is because indexing is most likely turned off on the network volume.

I tested this with an SMB Share to a Linux System and copied over some photos that I know contain GPS Info.

On first query I got the same results you did, a subset of the metadata, however upon turning indexing on using mdutil and letting the volume be indexed, then mdls returned all metadata including the GPS Info.

In Terminal use the following syntax, mdutil -i on mount_point, e.g.:

mdutil -i on /Volumes/Data


Also, you might want to read the manual page for: mdutil