Mind you this is a subjective, philosophical topic on what we each perceive constitutes one as god or a god. There are a multitude of defintions and that's what I want to hear.

Deus was originally designed as a interplanetary invasion weapon by mankind. It went out of control after its power source merged with the Wave Existence and after this it ended up on a ship which was crashed into a planet. To eventually resurrect itself, it started civilization and life on that planet. Its closest devotees and indeed parts of itself, the Emperor Cain, the Gazel and Miang, were the leaders of the people for its entire history.

So, he is the Creator there. But he is also a being created by man...I think there was an old book about this with an AI who created a universe.

So does he constitute as god or a god as is stated through 90% of the game?

Technically, since Deus is the one that started life on the planet, i would say he's a god. It's a god of the planet which Xenogears takes place on. But obviously, he's not the God who created the universe (I'm merely stating he's not "God" as depicted in Christian religion, that doesn't mean I really believe in it).

So, in my opinion: it's a god of that planet, but not the God.

I agree with Yorix that he is the God of that planet. Also in terms of him being man-made there are tons of references to ancient culture in Xenogears. It's possible that there may not have been God before he was created. But since that ancient culture was decimated, Deus basically created a rebirth with a god-like Genesis, so for the people of (don't remember Xenogears's planet's name) he is their Creator

Perhaps it's a jab at the whole concept of Godhood.

Deus creating humans and stuff on that planet made it a God.
But, that would mean that in a sense, the people who built Deus themselves created a God.
Who's to say that, after so many millions of years, the people inhabiting the world of Xenogears would've built another Godlike machine?