Who (exchanges/wallets) supports bitcoin gold fork/airdrop?
As far as i know the only below mentioned exchange & wallet provider is announced by the press time that they are supporting the bit coin gold fork by providing the equalant amount of bitcoin gold coins to existing bit coins after fork is taken place..
1# Bittrex (www.Bittrex.com): 
This exchange is supporting the bitcoin gold fork/airdrop is occuring on October 23, 11pm PT (6am UTC)by providing the equal-ant amount of bit coin gold coins to the bitcoins available on/before fork ,. See the below statement is released by the bittrex team.
" 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. BTC held on orders will also be credited. Only the BTC held on your account is eligible for BTG. Full details below.
Bitcoin Gold [BTG] is a proposed digital asset.
At this time, the information and codebase presented by the Bitcoin Gold team is not in a functional state and Bittrex will not commit to opening a Bitcoin Gold trading market.
Bitcoin Gold intends to perform a wallet balance snapshot at Bitcoin block 491,407 but the actual Bitcoin Gold network will launch publicly at a later undetermined date and time (the only guidance is the statement "after the launch on Nov 1").
Bitcoin Gold does not currently have:
Fully formed consensus code
Implemented replay protection
Adequate code for testing and auditing
Publicly known code developers
Bitcoin Gold codebase also contains a private premine of 8,000 blocks (100,000 BTG). Please be aware that if a market does open there is a possibility of the developers selling their premined BTG on the open market.
When considering adding tokens to Bittrex exchange we look at the risks involved. Bitcoin Gold does not satisfy our criteria for safety for our users. Taking a snapshot of the Bittrex exchange Bitcoin wallet is also a large disruptive undertaking that requires over 24 hours of preparation (Bitcoin deposits and withdrawals are also suspended during this time) and should not be taken lightly.
Bittrex will close deposits and withdrawals of Bitcoin (including 2nd layer protocol XCP/OMNI and associated assets) 24 hours prior to the BTG snapshot block 491,407 occurring approximately on October 23, 11pm PT (6am UTC). The wallet will remain closed until the on-exchange snapshot is complete. Trading will remain active during this time.
Bittrex will make all reasonable attempts to credit our users BTG but until we have a fully supported wallet, we can neither guarantee tokens or consider creating a market. We will continue to monitor the state of the codebase and network. We'd also like to encourage the BTG development team to reach out to us."
Link: https://support.bittrex.com/hc/en-us/articles/115002320451-Statement-on-Bitcoin-Gold-BTG-
2# Ledger Wallet (hardware wallet provider):
They tweeted on Oct 21 that they will support the bitcoin gold fork and allow the customers to get equalant amount of bit coin gold for existing bitcoin is available on/before fork. See their's tweets below.
Ledger Tweets;
Ledger @LedgerHQ Oct 21
Bitcoin Gold will fork on block 491407 - just hold your BTC in a Legacy or Segwit address before. Nothing else necessary.
Ledger @LedgerHQ Oct 21
We'll offer support to transact on the current and Segwit2x chains - with a dedicated website to split as there won't be replay protection
Ledger @LedgerHQ Oct 21
Ledger will add immediate support for the Bitcoin Gold fork if/when the code is available, with replay protection, and successful/valid.
https://twitter.com/LedgerHQ?ref_src=twsrc%5Egoogle%7Ctwcamp%5Eserp%7Ctwgr%5Eauthor