I think that running off of the drain would be problematic because you'd potentially get air bubbles into the reactor. Not a big deal every once in a while, but continuously fluidizing the GFO would be bad.
Anyway, running raw tank water into the GFO reactor would not be the most efficient way to use it. Ideally you'd run it with water post calcium reactor, if you have one, and if not after a protein skimmer, mechanical filtration, or refugium. It will have a longer lifespan if it is used with water that is sediment free (to prevent clogging) and low pH (to prevent calcium carbonate precipitation on the GFO granules). If you do use activated carbon, water should go through GFO first and then the carbon.
Hope this helps.