Also, there was a great post on /r/bitcoinmarkets by the CTO of another exchange, picking apart Gemini's technical setup. Worth a read if you're into modern frontend web development.
https://www.reddit.com/r/BitcoinMarkets/comments/3nkxh3/gemi...
Gemini is a spot exchange (simple buy/sell) while we're a derivatives exchange with much more complex requirements, so you'd expect a different set of decisions and tradeoffs, which is what I found. Gemini's real value (at this point) is in its ability to navigate regulatory capture, not necessarily in its technology. But their technology is a cut above what you usually see in Bitcoin exchanges. The exchange landscape has been plagued with unreliable/buggy exchanges, like the late Mt.Gox and the still-limping Bitfinex (which is much more complex).
On the whole, basic spot exchanges without leverage are relatively easy to create. I would love to do a more complete analysis but of course I don't have any inside information. I would be very interested in their backend, which appears to (possibly) be Scala. No clues as to whether they're using a SQL database or something more specialized like KDB+, which we use and love.