Gemini Exchange Policy In Case of Split.
In the event that Gemini detects a chain split or anticipates an imminent chain split, they will temporarily suspend BTC deposits and withdrawals. Gemini will endeavor to resume deposits and withdrawals as soon as it’s safely able to do so, but that process may take up to a week to complete.
What Gemini is currently planning:
The “BTC” balance and trading activity users see on Gemini will likely reflect the chain with the greater total difficulty.
Gemini may choose to eventually open new order books for the chain with less total difficulty.
Gemini will make every effort to support withdrawals of the chain with less total difficulty if it ends up having a significant amount of hash power, exchange volume, or economic value. However, unlike with the Ethereum / Ethereum Classic hard fork last year, Gemini cannot yet guarantee that it will succeed in this effort, due to a lack of two-way replay protection in the various forks of Bitcoin Core (including Segwit2x, UASF, BitcoinABC, and Bitcoin Unlimited).
The Gemini team finished its update saying:
“We understand that the policies listed above leave quite a lot to Gemini’s discretion. We would rather have clearly defined policies in place right now; however, it’s unclear what will happen on the blockchain in the coming days, weeks, and months, and we would much rather play it safe than making hasty, ill-informed decisions and risk mismanaging your funds. As soon as we gain further insight on the situation, we will make additional blog posts to better detail how your funds on Gemini will be secured.”
https://www.cryptoninjas.net/2017/07/20/gemini-releases-policy-upcoming-bitcoin-chain-split/