Bittrex is the one that promised to give you BTG coins against the BTC that you will be holding during the snapshot block that will take place at 491,407... If you would like to find more info, you could get it here: https://support.bittrex.com/hc/en-us/articles/115002320451-Statement-on-Bitcoin-Gold-BTG-

P.S.: The block was already generated yesterday so it's of no use as snapshot is already taken...