This has happened to many people. Including me. And PokeCinema.
But the truth is, Banjirou doesn't actually give you the Shiny Gible in the Battle Tower.
To actually obtain the Gible, you need to go to the second town in the game, after Aspertia City, and go to Alder's house. Banjirou should be standing next to Alder, so simply walk up to him, and take the Shiny Gible.
Trust me, this has confused many people.
You do need to get to the top of the tower before and fight him.
It took me 2 hours to get to the top of that tower. No joke.