Since Coinbase has $248M daily volume on BTC/USD, compared to $11M on Gemini, you'd probably want to sell on Coinbase. But placing the full order on Coinbase, even as a limit order, is also suboptimal. You'd want to ratchet down your trading fees, and then trade the largest amounts at the lowest fees. To avoid explaining maker & taker fees, I'll just take the average of the two.
Coinbase regularly (daily?) calculates your trading fees based on 30 day trading volume. So the best route is to sell the minimum needed to reach the next tier, wait, and then repeat.
With no trading, you're paying 0.50%
Sell $10k worth (0.4 BTC), and you now pay 0.33%
Sell $50k worth (2 BTC total), and you reach 0.20%
...
After selling $15M worth (564 BTC), your fees drop to 0.11%
You can sell $75M worth and wait for 0.075% fees, and then sell the rest
These last orders may need to be limit or iceberg orders spread over several days, since the amount being sold is relatively large compared to Coinbase's daily BTC/USD trading volume. Stock brokers have a trading desk to help with situations like this, but I don't know if Coinbase has something like that.
The numbers I used, and explanations of maker vs taker:
https://help.coinbase.com/en/exchange/trading-and-funding/exchange-fees[ Disclaimer : I'm currently shorting $COIN stock, but I have nothing against Coinbase. It's a play on crypto crashing when stock markets crash, and its my smallest position ]