Oops! Have I just disturbed a hornets' nest of apologists for papal infallibility? 'The document attributed to Gelasius MAY be of later authorship.' A poor defence. And 'the document refers to an apocryphal book titled something about the assumption of  Mary and doesn't address the concept itself. ' Actually, it does, at least obliquely. 

Let's try some common sense here: the fact that this doctrine, which Pope Gelasius did declare as heretical, would await many centuries for that solemn declaration by Pope Pius XII in 1950 does support the view that the doctrine had for long been considered heretical and that, therefore, nothing more was to be legitimately said about it.