When your withdrawal transaction is first posted, it's in mempool but not mined in yet. In a distributed system, it can take a few moments for the changes to propagate so that your transaction shows up everywhere. If one Ethereum blockchain explorer is slow to respond, another should be working.
No one blockchain explorer is up to date 100% of the time, so if you see your transaction "spinning" on one, look for your transaction hash on another one:
However, during certain particularly exciting periods - like token offerings - sometimes all the blockchain explorers are behind. During these periods of very high activity, your withdrawal may not show up in a blockchain explorer for several hours.
Please be assured that Gemini closely monitors outgoing deposits to make sure that your transaction will be posted and mined in as soon as possible.