When we did the EN -> JP renames, I did a lot of the work based on a Pokemon character list. In hindsight, I don't think the current tag name makes a ton of sense, as it just cuts off his last name of "Gropius" arbitrarily.
While the name could be changed to ghetsis_harmonia_gropius, I actually can't think of a single time his full name is mentioned in the games or anime (feel free to correct me). Bulbapedia only cites Masuda mentioning it at a fan meeting. I think the most recognisable tag name would be to just go with "Ghetsis".