To get free Bitcoin "gold", one just need to keep Bitcoins in exchange? Like for exemple did Kraken.com last time give Bitcoin Cash to all those who had Bitcoins? Relatively new in this world so don't know what happened in last fork. Or one need to keep them in particular bitcoin wallet to get free gold?
You shouldn't really hold your bitcoins in a exchange in order to get bitcoingold, because you simply don't know if the exchange will accept the new coin or not. With bitcoin cash I think things went well for most users, but those that did not hold their private keys were at risk of net getting any coins for the fork. I think you should transfer your coins to a wallet, where you own your private keys. This way you know you will get the bgold for sure.
I just want to learn that when this Bitcoin Gold will be available for Buying and Seling on the markets such as Bittrex and Poloniex.
I don't think they have announced it yet. Maybe you should try to see on the bgold website, what exchanges already support them. Last time I checked, those two weren't there yet. Not saying they wont list it, but right now, we don't know if they will.