BTC transfers will post to your account after 3 confirmations on the blockchain, which typically takes an hour or two to complete. You should see a notification of the pending transfer in your activity feed on Gemini -- the sidebar on the right of the Gemini screen -- after your transaction has one confirmation on the blockchain.
BTC deposit delays are not influenced by Gemini -- instead, the external wallet or exchange that you have withdrawn the BTC from maintains discretion over the fees that they pay on your BTC transfer to ensure timely arrival at your destination address. That being said, the blockchain has experienced very high traffic and congestion in the past 48 hours, so this is likely the cause of your delayed transfer.
To troubleshoot where your transaction is, try comparing the fees you paid against the current fee rate to be mined in within two blocks with 90 percent confidence. There are several fee models available, you may refer to https://bitcoinfees.github.io/ if you like. Another source of information is Blockcypher, which offers a full blockchain explorer at https://live.blockcypher.com/btc/. You may look up your transaction in the blockchain explorer and see if your fee rate falls within Blockcypher's current fee estimates for high priority, medium priority, or low priority.