A God Titan are a rank of titan that eats Lesser Titans for breakfast.
There is currently only ONE god titan in existence, that being Witherzilla, although there will be another one soon called the Executor Dragon, which, will be stronger than the current Witherzilla, although Witherzilla will be buffed after the release. What we know is Witherzilla has 10 million health, while other God Titans will be over 10 million health and probably would eat the current Witherzilla for a living. The only titan the can Damage Witherzilla is the Ender Colossus using its scream.