TOMI means wealth and rich. This kanji is commonly used for Japanese family names and you will see other names including TOMI, like Tomita, Tomiya and Tominaga.
OKA means a hill. This is also a common character for Japanese family names. The name Tomioka is very normal one for a Demon Slayer’s character as most of main characters have eccentric and rare names.
This character represents the right way as a person, a stance of acting reasonably with no personal greed.
YU means brave and courage.
Both of kanji characters in Giyu’s name have serious and upstanding meanings. I think it’s an eligible name for a member of the Kisatutai (the demon slayer corps) ⚔