I dont know Japanese, but I know a little bit about hiraganas and katakanas. The "Ji" (spelled as "G" in English) is in the group of the letters with "Z" sound in them (Za, Ze, Ji, Zo, Zu). They use a smaller version of the "Y..." characters to modify this "Ji" letter, so the "Ji"+"Yo" is spelled as "Jo". The official English romanization it's either "Jyo" or "Jo", but maybe there is a non-English romanization where its "Zyo".