I will put in my 2 cents, Bill.  The term "homoousias" which was declared to be the first major creedal symbol of the Christian Church at the Council of Nicea in 325 AD had been condemned in 247 AD at the Council of Antioch as the symbol of the Sabean heresy.  It was used by these heretics to imply a modal identity between the Father and the Son.  Resistance to the use of this word by many bishops after Nicea was not due to Arian sympathies, but to their concern that this phrase had been previously condemned and was proximate to heresy.  When we say in the Creed that the Father and the Son are "of one substance" we do NOT mean that they are identical as the Sabeans claimed but that they share a common divine nature.  So the term "substance" (Gr. ousios) can have several subtle differences in meaning and cannot be considered a univocal term.  With regard to Pope St. Gelasius' statement, he uses the term "substance" in a phenomenological way in the same sense that we describe a "sunrise" when we know full well that the sun does not rise, but that the Earth rotates.  He use the word substance to describe the physical presence of the Eucharist as it appears to us.  Please note that this sentence is the ONLY reference by Pope St. Gelasius to the "bread and wine" of the Eucharist.  Indeed he refers to the substantial characteristics of bread and wine in contrast to the image and likeness of the Body and Blood in the Eucharist.  This is an ironic and dialectical comparison.  We would likely reverse that terminology and speak of the substance & nature of the body & blood and the image & likeness of the bread & wine.  I think Pope St. Gelasius' used the Greek understanding of symbolism which is much stronger than that of the Latin West and which underlies the theology of the Icon.  To the Greeks, the image of the Eucharist reveals God's presence while the substance is merely how it appears to our senses.