Even both the Poloniex and Bittrex didn't announce anything about Bitcoin Gold whether they are supporting or not. But I hear the news from Yobit they are supporting Bitcoin Gold.
https://support.bittrex.com/hc/en-us/articles/115002320451-Statement-on-Bitcoin-Gold-BTG-If you have a Bitcoin (BTC) balance on Bittrex during the BTG snapshot block 491,407 occurring approximately on October 23, 11pm PT (6am UTC), you will be additionally credited the equivalent amount of Bitcoin Gold (BTG) on a 1:1 basis. i.e. 1 BTC on Bittrex held during the on-exchange snapshot will get you 1 BTG.