Besides being a Dunbine reference, it's also a nod to the originally planned RX-78-2 that Tomino first attempted in the original Gundam before he was told to add more colors.
Also, it already stablished that there's "newtype magic" going on with zeknovas and Lalah creating new universes. A giant Gundam from the other side shouldn't be too crazy, all things considered.
The biggest mystery to me is who was Shuji and why was he able to fly like a space ghost but then be completely normal and not a ghost at the same time.

How much HP will it have?

>final boss is the originally conceived grey/silver gundam Tomino envisioned
I honestly kneel