i got the key for black city, and I much rather have the gible than a dratini, cuz I already have 3 EV trained dragonites (first world problems)
is it not possible to get the shiny gible instead of the dratini in Pokemon white 2?
Actually, yes, but you need a separate Black 2 version that has the Black Key. Then you need to swap the keys.
If you're not lazy, then you can just get a Gible and breed to make it shiny.
No you can't get a shiny Gible from Benga in White 2., definitely not if you've already completed White Hollow and got a Dratini from him (he's not going to give you 2 shinies!!).
If you have a friend who has Black 2 either trade your Dratini and his/her Gible or ask them if you can breed their Gible (breeded Pokemon are more likely to be shiny if their parent is shiny).
Besides, the shiny Gible/Gabite/Garchomp looks barely any different to their non-shiny ones so I don't really think it's worth the trouble.