Upvote:3
Nice question. According to C.Martinell's book
GaudΓ: his life, his theories, his work. β Cambridge, Massachusetts: The MIT Press, 1975.
pages 116-118, nobody kew, at first even in the hospital where Gaudi was taken after the accident. I do not see any reason to doubt Martinelli's account:
P.S. Top-x lists normally should be taken with huge grain of salt, but in any reasonable list of most famous architects in history, Gaudi will be in the top 10. See for instance here, or here, or here, or here. He was definitely, the most famous architect in Span, ever.
Edit.
In my answer I interpreted the question as about the reaction of bystanders at the time of the accident. I have no insight into who they were and what their reaction was after they heard the official news few days later and realized what they had witnessed earlier. Since, according to the book, cabbies were fined later on for refusing to transport Gaudi to the hospital, at least one of the eyewitnesses contacted the authorities after the accident, which might explain how this person (or these people) became known.
According to the excerpt from the book above, Gaudi was registered as Antonio Sandi at the hospital, which suggests that if he did identify himself at the time of the accident, his name was garbled/misunderstood. But this is just one of several possible explanations.