Gyokuken is not a widely recognized canon character name in Jujutsu Kaisen, and there is no clearly established full name for a major character by that exact name in the series. If this refers to a minor, fan-translated, or alternate spelling/name variant, the identity may be ambiguous. In Jujutsu Kaisen, characters typically have clearly defined Japanese names, so this entry should be treated cautiously unless you can provide additional context such as an episode, manga chapter, or alternate spelling.