It is summoned when the Godzilla event has started or when a player places his egg on a block. To start the Godzilla event, you can do "/event start Godzilla." However, you will need to prepare very well beforehand.
He has 10,000 HP and heals 10 HP a second. He can also do a lot of damage (too much for me to calculate).
If you are wearing Iron Man Armor, look in between his feet to see his stats if you are tracking with JARVIS.