My name is GUNDAM TANAKA! Remember it well, for it will be the name that will one day belong to the Pokémon Champion!
Dedenne: You can't talk about Gundam without mentioning his Four Dark Devas of Destruction, and Dedenne is the closest possible thing there is to the hamster Pokémon required to do his Devas justice. Ironically, as a Fairy-type, Dedenne actually excels at defeating those who dabble in the dark arts rather than assisting them.
Arcanine: Legend says that if you can properly tame an Arcanine, it will become fiercely loyal to you and never leave your side. This kind of challenge would likely be nothing to one such as Gundam, who I imagine would proudly use Arcanine in battle as proof of his mastery of animals.
Greninja: Aside from being Dark-type, Greninja also seems to share some of Gundam's fashion sense; sporting a long, pink scarf. It's also top-tier in the competitive field, symbolizing Gundam's top-tier status as a character.
Accelgor: IF YOU FLINCH, YOU WILL DIE! As one of the fastest Pokémon around, Accelgor represents Gundam's craftiness and quick wit. It also pretty much looks like Gundam in Pokémon form.
Zangoose: Zangoose are known to be loners among Pokémon, preferring to spend time alone or with others of their kind rather than socialize with different species. This is similar to how Gundam considers his supposedly-inhuman status as an excuse to seclude himself from the rest of the group. Zangoose also shares Gundam's awesome facial scar tattoo.
Mega Houndoom: Can you think of a better Mega Pokémon for Gundam to train than Mega Houndoom? Representing Gundam's darkly heroic nature, Hellhound Earring and INFINITY UNLIMITED FLAME (supposedly a burn from a Houndoom will hurt until the day you die), this sick puppy's the lynchpin of Gundam's Pokémon team.
So who's next? You decide! Whoever gets the most votes by this time tomorrow will be given the PokéRonpa treatment.