I suspect this guy is "Yuichi Kanatani", who has been credited in romaji in this
album. JASRAC reveals the kanji are 金谷 裕一. He(?) was also credited in kanji for some minor anime songs like Prince of Tennis's Wonderful Days.
I can't find a definite link between Ryu ga Gotoku's 金谷 裕一 and that Yuichi Kanatani, but I don't think 金谷 裕一 is a common name, so...