I have bitcoin in blockchain.info (where i control the private keys) before 25 Oct.
Which exchange/wallet can i use to split the bitcoin into BTC and BTG so that i can sell them separately before 1 Nov 2017?
At the time of the snapshot, the coins which you controlled in your wallet would exist in both the original BTC blockchain, and the newly split BTG blockchain.
You can redeem these BTG coins for BTC (or another Cryptocurrency) via an exchange which supports these tokens (eg Bitfinex, Yobit, etc) once the BTG blockchain is being publicly mined, and transactions can begin.
Selling before this time would have to be done via a futures contract, although I do not believe there are any major platforms trading BTG futures at this time.