I have tried adding a 'description' with Google Photos on a file which I imported.
I suspected that Photos did not use the same exif or IPTC metadata field than Elements. So, I checked with Exiftool GUI what was in the file itself, and I don't see any mention of the 'description' in that file.
Elements only reads the data in the file itself, not what is stored in Photos. Maybe it's the same as the tags which are only kept in Elements catalogs unless you voluntarily 'write metadata to files'?
I tested the reverse: importing into Google Photos a file with a caption entered in the organizer does work. The caption is in the info panel of Photos, but the tags are not imported.