Rowling said about the word "Gringott":
Gringotts, really, I think, came from Ingots. You know, you get ingots of gold, those bars? So I just liked the sound of it, so to me it sounded, 'gr' words can sound quite aggressive or quite or even sinister. So I really combined Gringotts. I just thought it sounded that little bit intimidating, but it had that allusion to gold in it (ITV)
The date of the founding of Gringotts comes from a plaque in the Diagon Alley area of the Wizarding World of Harry Potter (WWHP). While we can't be absolutely sure that this information came from Rowling, our sources tell us that she did in fact create the factual information in the park.