I know Shanks found it but Devil Fruit do reappear after the person who ate it dies. So is it known if anyone ate the Gum-Gum Fruit before Luffy?
Shanks knew about the fruit from the "Devil fruit encyclopedia" (悪魔の実図鑑). Oda answered this in the SBS Volume 45:
D: Hey, Odacchi! I got a question! About Devil Fruits!! If Kaku and Kalifa said they wouldn't know what their Devil Fruit were until they ate it, how come Shanks knew what Luffy's Gomu Gomu no Mi was before he ate it?? Tell me! Tell me now!! Pleeeease tell me!!! P.N. Shirogitsunekko
O: Well, I think I'll eventually get to explaining more about Devil Fruits in the main story, but you see, there's a book of Devil Fruits. It has info about all kinds of fruit names and abilities, but few of the fruit can actually be identified by their shape. In the case of the Gomu Gomu no Mi, there was a picture and everything, but for Kaku and Kalifa, they were talking about how they wouldn't know until they had finally eaten the fruit. That was in Volume 40.
Since it was in the book, we can conclude that someone must have already eaten the fruit before.
Probably but we do not know. We have not seen any other reference to someone else with that power. We do not yet know the origin of devil fruits enough to know if new unique standard fruits can naturally form. We do not know if detail where Shanks obtained the fruit.
Shanks was aware of the name of the devil fruit immediately after Luffy ate it. It is possible he could guess the name of the fruit from it's appearance, from a brief display of Luffy stretching (various ways based on source material), or from some exotic method of obtaining the fruit (aka Vegapunk). Far more likely, however, he knew the name because someone else once possessed that power.
As with your other question there are fan theories but those are not on topic. We do not know if I am right and definitely do not know who that person could have been.
No we don't. Though the anime and manga have multiple adaptations of how Luffy obtained and ate the fruit the cannon of the franchise does not mention if there was a gum-gum user before Luffy.
A lot of information about Devil Fruits, their origins and purposes slowly unveils through different key characters and details about the story which is why the devil fruit is a key plot device.
Right now I'd go as far as to speculate that we will possibly learn the answer to your question as the poneglyphs are further explored in Luffy's current quest for Rafftel and the struggle against the Yonko.
Most likely there were many. Some people like to think that it's a special fruit that was once used by someone really strong like Roger himself. I feel like that would've been mentioned already and more importantly I like the idea that Luffy is the first to use his devil fruit in such impressive and innovative ways.
For me, i think that shanks know about this DF. But it's not a special or something. Oda mentioned that he wants to make luffy a character that will not make the audience bored. So he gave the gomu gomu no mi to him as you can see he can stretch and make fun about his body but at the same time, he can be a battle genius to utilize his DF to full potential in combat. Its 2 in 1. Being a battle genius using DF and haki combined is based on character, Not the fruit itself. Which means that if someone in past already ate this fruit. He/she probably can't do what luffy do with the fruit now. Thats make this fruit not special at all. So the conclusion is, gomu gomu no mi is just a regular fruit but it become a strong fruit because of the user know how to use it properly.