You know, everyone involved in the Gemini program seemed to love that spacecraft -- including but not limited to the guys who flew it.
But Gemini showed a pretty large maturation curve during its manned flights. There were lessons learned during Mercury that took a while to incorporate into Gemini, not the least of which were manufacturing lessons. And if it seemed to take a while for McDonnell-Douglas to learn from its own mistakes, think of how much harder it was for North American Aviation to learn anything from Gemini as it designed and built Apollo.
However, there is an element of heightened danger in flying a Gemini spacecraft that far from home that I think most people underestimate. While Gemini was a much safer spacecraft to fly by the end of the program than it was at the beginning, it suffered from a far higher malfunction rate than the Apollo flights eventually logged.
For example, one reason NASA didn't attempt flights longer than three or four days after the Gemini V and Gemini VII marathons was that a majority of the attitude control system thrusters had failed by the time each of those long-duration flights concluded. On Gemini V, in fact, the thrusters were in such bad shape that the crew was told the activate the re-entry control system two orbits prior to retrofire, to ensure positive control when setting up retrofire attitude.
The Gemini fuel cell system was notoriously finnicky, and always seemed to totter on the brink of reliability without actually achieving it. They didn't produce potable water, either -- the water those cells produced was full of an organic sludge euphemistically called the "brown fuzzies" by the engineers. If the cells put out too much water (as happened on one flight), you had a serious problem, since the nasty fuel cell water was used to pressurize the potable water tank. On a lunar flyby, such a problem could lead to a waterlogged outbound crew and a really thirsty inbound crew... not to mention the need to perturb the outbound trajectory frequently with "Constellation Urion".
Finally, the Gemini wasn't designed to handle the thermal loads of constant sunlight for several days at a time. Granted, Apollo needed to do passive thermal control rolls, and Gemini could have done the same thing -- but these kinds of thermal effects hadn't been considered when Gemini was designed. This issue alone might have ended up killing the idea, had it gotten that far.
And remember, you'd have to dock with a much bigger propulsion module than a Centaur or a Titan transstage to do more than follow a figure-eight loop-around of the Moon with a Gemini. Yes, it would have been an historic moment, and the Russians fought hard (though ultimately unsuccessfully) to pull off this very type of mission. But compared to what Apollo was capable of, and *designed* to do, trying this stunt with a Gemini was simply an unjustifiable risk.
-the other Doug -- Senior Member