Read between the lines, Joan. Sometimes historical research can occur only by circumstantial evidence rather than direct proof, in other words, 'by deduction from known facts'. 

It is a reasonable assumption that Gelasius rejected the book because of its very subject matter. Had he accepted the doctrine of Mary's assumption, then why didn't he declare this? He never did. And he must have known that declaring the book apocryphal risked, among Catholics in general, an equal judgement on the book's topic. The fact that Gelasius appears to have made no distinction between the book and its content suggests his utter disbelief in the doctrine itself.

These ideas of mine are reinforced by the fact that Pope Hormisdas, in the following century, declared that anyone who taught this doctrine was a heretic.

And your comment (that I had no idea why Gelasius declared the book heretical) does indeed imply that you know the answer to this. How else could you reasonably say of me what you did?