Gyoubu is definitely an older brother figure for Genichiro. He helped raise him and probably taught him skills from being a bandit that a noble’s grandson most definitely shouldn’t know.
Lock-picking. How to tell weak points in structures, like windows or doors, to break in cleanly or looking for the weakest point to brute force their way through. Probably how to jury-rig a makeshift explosive, because, hey— it might come in handy.
He gives me the impression him and Genichiro used to go camping, or the equivalent back in the day, for fun when Genichiro wasn’t busy training or studying. Hunting and surviving in the forest for a few days just for fun or to get away from the castle. Isshin was probably worried sick waiting for his grandson and Tomoe would probably have to help distract him.
Gyoubu was definitely the one to teach Genichiro to ride a horse, both with a saddle and reins, and without. Genichiro was probably scared of Onikage at first and Gyoubu would have to tell him that a horse can sense your emotions and will react to them, along with your heartbeat if they consider you part of their herd.
Tomoe taught Genichiro both archery and swordsmanship, along with Isshin in the swords part (that man can only shoot a gun and is a horrid shot with a bow). Gyoubu probably taught him hand-to-hand combat instead. The most basic way to defend himself if he was in a situation his katana didn’t have space for or his bow was missing. They definitely both ended up with bruises once Genichiro got older and better, leaving poor Dogen or Emma to scold them if they accidentally broke a bone or too purple a bruise. Roughhousing and whatnot.
They just give sibling vibes. Maybe Gyoubu was the only one that took Genichiro wanting to learn lightning seriously and helped him scheme in how to convince Tomoe after making peace Isshin would most definitely kill him for giving Genichiro the idea.
Perhaps Gyoubu being a bandit was a motivator and a role model for Genichiro whom was adopted and was probably struggling to fit in for a long time. So, ¡older brother status acquired! Helping Genichiro grow out of his shell. Genichiro seems like a person that would say what he means bluntly with almost no regard for your feelings, and that would probably come from Gyoubu who used to be a bandit years before he was yanked into the Ashina Clan by Isshin.
Honestly I love Gyoubu despite how hard he kicked my ass the first time I fought him. I killed Lady Butterfly first try before I even came close to killing him. Idk what it is with spears but I am horrible at fighting nearly everything with one.