Gyoku is sushi version name of Dashimaki Tamago (出汁巻き玉子). Kanji 玉 has 2way to read, tama in kyuyomi (訓読み), gyoku in onyomi (音読み). Gyoku was originally a sushi chef's jargon. They like short names. Dashimaki Tamago in lunch box, it is a Japanese Omelette. Japanese never call it “Gyoku”