If memory serves, I thought I remembered reading somewhere that the GTS Pokemon would be filled with all the Pokemon names. But when I'm searching for one that I haven't seen before, for example a Dragonite, I don't see it in the "D" list.
But there is an option you can pick to where you can type a Pokemon name and it will search for it. So can you search for Pokemon using that option that haven't been seen in the Pokedex? Also can you shorten the name to search for any pokemon with what you're looking for? Say I searched for "Drago", would that find Dragonair and Dragonite?