What brings Gemini and Capricorn together is rationality, detachment, analysis and comedy. What sets them apart is more or less everything else. Capricorn, solid as a rock, feels the constant need to parent the youthful and irresponsible Gemini, and perhaps try to dictate their behaviour. Gemini respects Capricorn but won't tolerate too many do's and don'ts, and just cannot believe the dull single-mindedness of Cappy. Capricorn may be impressed by the quick wit and self-mocking attitude of Gemini, and there could certainly be sexual attraction based on the thrill of what one can't understand.
It would be interesting to watch Gemini and Capricorn behind closed doors, for while both can be quite reserved romantically and emotionally as part of their public image, neither is likely to hold back when it really comes down to business. An interesting flirt and sexual combination, but in most cases with too little natural compatibility for things to work.