You need to have defeated the Elite Four and the Champion if you want to get a Shiny Gible. After that, you have to go to Black City and enter Black Tower (The very tall building) and clear every area. There are 10 areas in total. At the end, you will meet Benga. He has Level 80 odd Pokemon, so you should train before battling him. After you have defeated Benga, go to Floccesy Town and he will be in Alder's house. If you have an empty space in your party, he will give you your Gible.