I'd go with Dratini. While Gible's Attack and HP are a little bit higher, its Special Attack and Special Defense are lower, and it's Ability isn't as useful (in my opinion). Gible's Sand Veil Ability gives it a nice evasion boost, but it only works in a sandstorm. You could teach Gible Sandstorm to take advantage of its Ability, but, chances are, it would also hurt your teammates. Being able to get rid of status conditions with Shed Skin can really help out Dratini a lot. Also, Gible's 4x weakness to Ice can be crippling. Sure, Gible gets a resistance to Electric, but it isn't resistant to Grass or Water like Dratini is. If you planned to evolve your Gible/Dratini into a Garchomp/Dragonite, I'd still go with Dragonite. It can't flinch due to Inner Focus (which isn't as useful as Shed Skin in my opinion), and Garchomp still has Sand Veil. Garchomp has two weaknesses, Ice and Dragon, while Dragonite has three, Ice, Dragon and Rock, so Dragonite has an extra weakness, as well as a 4x Ice weakness, sadly. However, Dragonite is more bulky, having 95 Defense and 100 Special Defense, while Garchomp has 95 Defense and only 85 Special Defense. So I'd go with Dratini/Dragonite, since Dragonite's more bulky, but in the end, it's up to you.
Hope I helped!