I have been lucky enough to have visited Barcelona several times, most notably in 1968 and 1984, before everything was cleaned up for the Olympics.
In those days, the attitudes were more relaxed, and I was able to wander freely throughout many Gaudi buildings, climbing the highest towers of the Sagrada Familia, ducking up onto the roof of one of his apartment buildings, sticking my head into the private lobby of another.
There was a dusty display of Gaudi drawings and models in the base of the Sagrada Familia, but even better, in 84 my wife and I knocked on the door of the little house in the Parc Guell, and were given a tour of Gaudi's home by a woman who said she had been his housekeeper. Since he died in 1926, she must have been in her 80's at least. All his furniture, knickknacks, and lots of original drawings and models were still there, seemingly as he had left them. I would guess there had been some curation over the years, but it still seemed quite authentic, and the old woman told great stories.
He did a lot of modelling in plaster and clay, and a lot of suspended string models, which he used to develop arch designs, using gravity to make the curves. Then the masons would have to figure out how to replicate these, right side up, in stone and brick.
I dont know how much he did that we would call "sculpture" himself, though- I think he drew, and talked the artisans thru, a lot of the sculptural elements on his buildings, but I am not aware of him personally sculpting much larger than models. I know on the Sagrada Familia, there were several noted sculptors who, all by their lonesomes, carved sculpture after sculpture of saints and human figures, and Gaudi just designed the niches they went in.
There is no doubt Gaudi was a genius, but he worked within a time and cultural place when a fair amount of other, similarly talented architects were also building amazing structures. In Barcelona, the work of Jujol and Puig, friends and contemporaries of Gaudi, is equally amazing, just not as well known. And in Brussels, with Horta, or in Paris, or in Italy or Argentina, there were other great architects doing wonderful things.
There is an Art Nouveau skyscraper in downtown Montevideo, Uruguay, for instance, that is truly awe inspiring, the Palacio Salvo-
And you can buy an apartment there- for a mere $75,000
At the time, it was a unique cusp between the age of craftsmanship and the industrial revolution, and these artists took advantage of both to create art nouveau. There was a newly wealthy middle class worldwide that was willing to take chances, and a feeling of optimism of the new century that had yet to be dashed by the first world war.
In iron work particularly, something I study and practice, amazing things were done, when you could combine newly available technologies like power hammers, shears and punches, and oxy-fuel welding with master craftsmen who had trained their whole lives.