They felt like giving Ash a Dragon-type.

Going down the list, they find Gible. Gible seems like the perfect choice. He's a Dragon, he's got a unique typing, he can learn such-and-such attacks, etc. etc.

The reason they choose Gible, and not say, Gabite, is because they want to (theoretically) develop him in a meaningful way and it's easier to do so starting as a small pup. We've seen elements of this so far as confirmation, what with him getting to learn Draco Meteor and being slightly wild.

They've already given him Gliscor, a very uncanny decision, so now they're retreating to a more natural Pokemon.

And so, we come to our conclusion. We will probably never really see more than half the picture. Whatever underlying purposes in choosing Gible there are (like marketability) will stay unbeknownst to us.