So If now I export my private keys from electrum(even with 0 BTC on it) and import it to the futur BTG wallet. will it work ?
That's right.
Imagine something like this: You had 1 BTC in your wallet. At the moment of the snapshot, someone took a picture of the whole blockchain and saw that you had 1 BTC. Because of that, they wrote down that your address had 1 BTC and should receive 1 BTG after the fork. Since they already wrote this down, you are now free to spend your coins.
(I am wondering if electrum save your history in term of which BTC you own on electrum wallet before the forks)
What do you mean? You can see the history of your transactions in your wallet.