Bitcoin Gold fork is coming or did it already happen?
It's happening on October 25.
When i need to withdraw BTC from Poloniex to Ledger or some another wallet and how i know it's safe to put it back to Poloniex?
Before the fork happens. But try to do it as soon as possible to avoid delays with the exchange. After splitting the BCG from your Bitcoin address, wait a few days after the fork before depositing your coins back at Poloniex.
Is there any list which have wallets which allows user to control private keys?
Electrum is by far the best. Use it if you can.
Is BitGo good place keep Bitcoins during fork?
I wouldn't. A multi-sig wallet should make it more difficult to split your BCG and BTC from your address after the fork. If possible, create a Standard Wallet at Electrum and store your coins there.