The gothic churches as we know them are creations of the 19th century. They have been meticulously "restored," which is a modern and controversial concept. Before romanticism and the focus it brought to gothic architecture—as one of many aspects of the middle ages, the "age of romance"—these buildings were neglected, both in thought and reality: they were in other words very much dilapidated. When Victor Hugo wrote "The Hunchback of Notre-Dame" in 1831 Notre-Dame itself was almost a ruin. The seminal preface he wrote to the novel, while to a degree symptomatic of a growing interest in gothic architecture (which started in a gimmicky way in the 18th century), was extremely important in calling attention to these buildings—professional attention as well as aesthetic. He compared the gothic cathedral to the modern novel, in that it was what this society turned to in order to express their beliefs, dreams, and fears; it was their highest mode of communication—with each other and with their ideal, the physical embodiment of their scholastic philosophy, and an almost Pompeian preservation of their lifestyle. This doesn't have to be exactly accurate to be largely true, and anyway it was how his age came to study the style seriously. Ruskin in England held similar beliefs; again symptomatic of a growing—and cyclically almost inevitable—interest in the "gothic" but again he went far beyond conventional advocacy, and he set the terms of the discussion with his mystical belief in their communal meaning—in the religious, organic implications of their construction—as opposed to the standardization of classicism and the insincere adoption of pagan symbolism that comes with it.
it is not coarse cutting, it is not blunt cutting, that is necessarily bad; but it is cold cutting—the look of equal trouble everywhere—the smooth, diffused tranquillity of heartless pains.
The knowledge you refer to, then, was not preserved directly; it had to be rediscovered—through reverse-engineering and scholarship, digging up technical accounts of construction and anything that might be of relevance (there exists a detailed account of the building of Canterbury Cathedral, for example). Catalogues of ornamentation collected from all over Europe were put together. On the conjecture that eroded stonework was modeled on nature, the naturalistic study of nearby foliage was undertaken. The growing institution of Antiquarianism—which sprung up as quasi-official medium for nationalism in the 18th century—was essential to this documentation. The classical revival of the Renaissance had completely renovated the profession, and by the 17th century the gothic was neglected to the point of incomprehension (Italian propaganda against the German yoke they'd been under since the barbarian invasions played a part). For approximately 250 years—with some notable lapses—elegantly clear abstractions were the foundation of the art; the kind of orally transmitted, on-the-job training required for gothic orchestration requires a living, working tradition; a certain unconsciousness—communal absorption—in the task at hand was lost as modernity advanced. Some would say that to speak of the gothic as a "style" is anachronistic, as no conscious selection guided it; it was simply the way one constructed things. The "Middle Ages," and by extension its forms, were constructs of a post-renaissance modernity, self-consciously articulating everything in relation to itself—to everything it was not (this belief in the medieval hivemind is I think quite reductive). So once the demand ceased to exist, so too did the understanding (unlike the reducible formulas of antiquity), and it could not be resurrected without a self-consciousness perhaps fundamentally at odds with its origins.
the Goths, Vandals, and other barbarous Nations, subverted and demolished...that glorious Empire, where those stately and pompous Monuments stood; introducing in their stead, a certain fantastical and licentious Manner of Building, which we have since called Modern or Gothick. Congestions of heavy, dark, melancholy, and monkish Piles, without any just Proportion, Use or Beauty, compared with the truly ancient; so as when we meet with the greatest Industry, and expensive Carving, full of Fret and lamentable Imagery; sparing neither of Pains nor Cost; a judicious Spectator is rather distracted or quite confounded, than touched with that Admiration, which results from the true and just Symmetry, regular Proportion, Union, and Disposition; and from the great and noble Manner in which the august and glorious Fabricks of the Ancients were executed.