The GPS in .xmp standard seems to be like this:
<exif:GPSLatitude>36,36.36S</exif:GPSLatitude>
<exif:GPSLongitude>66,54.54W</exif:GPSLongitude>
That current Photos.app 6.0 .xmp flaw can be bypassed:
The metadata from those sidecar .xmp files can be copied to the corresponding .jpg files with GraphicConverter 11.3.3 > File > Browse... > [select folder] > [select .jpg files] > IPTC/XMP menu > XMP Sidecar File > Copy into JPGs, TIFFs, PNGs, webp, HEICs. GraphicConverter 11.3.3 handles this OK although some wrong GPS info is copied to the .jpg.
Correct info can be copied with the following exiftool (*) workarounds:
exiftool -overwrite_original_in_place -ext jpg '-IPTC:CodedCharacterSet=UTF8' -tagsFromFile %d%f.xmp -@ xmp2exif.args '-GPS:GPSLatitudeRef<XMP:GPSLatitudeRef' '-GPS:GPSLongitudeRef<XMP:GPSLongitudeRef' -@ xmp2iptc.args '-XMP-photoshop:DateCreated<XMP-photoshop:DateCreated' '-FileCreateDate<XMP-photoshop:DateCreated' '-FileModifyDate<XMP-photoshop:DateCreated' .
The workaround has the following changes:
'-IPTC:CodedCharacterSet=UTF8' added so High-ASCII umlauts in the IPTC metadata are correctly copied to .jpg files with blank metadata.
'-GPS:GPSLatitudeRef<XMP:GPSLatitudeRef' and '-GPS:GPSLongitudeRef<XMP:GPSLongitudeRef' added so the correct references are copied.
'-all:all' is omitted because it would copy erroneous references to .jpg but '-XMP-photoshop:DateCreated<XMP-photoshop:DateCreated' added because if such .jpg is imported back to Photos.app 6.0, it gets the date from the 'XMP-photoshop:DateCreated' instead 'ExifIFD:DateTimeOriginal'. Isn't it weird that it does not prefer DateTimeOriginal?
If that Photos.app 6.0 .xmp flaw is fixed, the following shorter command should work. Notice that older Photos.app versions export different .xmp and Photos.app 4.0 seems to have 'photoshop:DateCreated' and 'photomechanic:TimeCreated' date and time tags.
exiftool -overwrite_original_in_place -ext jpg -tagsFromFile %d%f.xmp -@ xmp2exif.args -@ xmp2iptc.args '-all:all' '-FileCreateDate<XMP-photoshop:DateCreated' '-FileModifyDate<XMP-photoshop:DateCreated' .
(*) The following pre-built settings files can be used as an aid to copy .xmp tags that Photos.app has updated to the original .jpg files. Notice that by default Safari.app wants to download a .html version so click the .args file you want > "Raw" > File > Save As... > Format: Page Source and strip .txt (or .webarchive that might be suggested) suffix OFF. Or use Firefox, Chrome or other browser to download a text-only version:
xmp2exif.args
xmp2iptc.args
https://github.com/exiftool/exiftool/tree/master/arg_files