Cleon
Legend
Magical Beast for me, too. The other types of gargantua should be the same type, and they don't sound like they'll have much in common with dragons.
I was thinking exactly the opposite, using different types for each.
Reptilian - Dragon
Humanoid - Monstrous Humanoid
Insectoid - Magical Beast (like a Spider Eater or Phase Spider)
Plus, there's the Gargantuan Carrion Crawler and Troll from the Mystara Appendix...
Troll - Giant
Carrion Crawler - Aberration
I guess we can't do the Carrion Crawler since it's Product ID, but is there anything stopping us doing the Gargantuan Troll?