Gluttony is said to be a fake Gate of Truth, as shown by when he swallows Ed and Ling. They are transported to a place that is full of blood, which seems to be entirely different from the place where the real Gate of Truth takes someone. What does him being a fake Gate of Truth mean, as opposed to just being some sort of transportation device? What is the difference between his Gate of Truth and the real one?
Gluttony was created for the sole purpose of being a portal of Truth. Father was attempting to create a way to access the so-called "non-reality" (beyond the Gate) without paying a toll; essentially, he wanted to entirely bypass Truth so that he could access all alchemical knowledge within the Gate.
However, Father is not omnipotent; he was unable to create a portal which was able to do what he hoped for. Instead, what you see of Gluttony was the result: A portal, attached to a homunculus, that serves as a gateway to a different world, one that is neither reality nor non-reality (that is, it is somewhere between the universe and the realm of Truth).
It probably goes without saying at this point that the difference between this Gate and the real one is that, well, this one is useless. Basically, it's just its own dimension that serves as nothing but a place for Gluttony's consumptions to go. It contains no alchemical knowledge, no God (which would be in the real Gate), and no way out.