Regifags, we're on Arceus' side. He was created directly by Arceus and serves as a physical master of the elements and helps to literally shape the world. On top of that, multiple other Pokemon were sourced as the Titans.
Just because he's a giant doesn't make him one of THE Titans. You're forgetting literally every other legendary sourced as the Giants as well as the fact we KNOW these Pokemon all worked together to aide the rest of the world.
The fact that Giratina isn't here and was sourced as one of the Giants, as well as the fact that the weather trio were made from the body of one of the giants, shows that not only were they not afraid to put names to these giants but that his name having gigas in it only relates to his size compared to the other Regis. He's 12 feet tall. Giratina, Palkia, Ho-oh, Dragonair, Lugia, Rayquaza, Dialga, TONS of Pokemon are taller than him and would fit better as the Titan.
Titans have a mythological origin as a class of divine being just like Gods and they were opposed.