Hello. I am a big fan of Fullmetal Alchemist, and I disagreee with rating it as Unknown.
Firstly, Truth is a conceptual being that embodies basically everything in existence. It is every single human, every single thing, every concept, and the entire universe. It is clear that it is Omnipresent, and I think it should also be Low 2-C, as by definition both space and time are part of it.
When an Alchemist attemps Human Transmutation, they always find themselves before
The Gate, and meet their perception of "The Truth", who more or less punishes them for attempting to break the Law of Equivalent Exchange. They are pulled into the gate, and are exposed to "pure knowledge about the world, the universe, alchemy and everything" and they break down, becoming one with the flow of everything. How much of said knowledge they are exposed to varies on the sacrifice the Alchemist gave, and since said knowledge is a part of "The Truth", it is clear that it is Omniscient.
Also, "The Gate" is different for every person, as it is the representation of their Truth and their understanding of The Truth. For Edward Eldric, The Gate was engraved with an image of the Sephirotic Tree of Life and held all the Truth about Alchemy. Basically, inside that Gate was all knowledge about Alchemy and all Alchemic Power, and that too is part of The Truth. When he gave it to The Truth, Edward becase simply incapable of using Alchemy ever again.
In Chapter 104, it is revealed that Planets and Stars also have their own Gates of Truth, as they are "living beings". This likely extends to even greater things, such as galaxies and even the entire universe.
Also, God Father should be Possibly Planet Level, as he absorbed what lied beyond the Planet's gate, which would logically be all that made the Planet's truth and being.