Gemini and Capricorn are signs of truly different nature. Ruled by Mercury and Saturn, they could form a relationship where they are both worried all the time. If it went that deep. Than they would be worried about each other, the faith of the world, whatever their main shared logic is. On top of that, Mercury is fast and agile, while Saturn is slow and stiff. Mercury is conscious, Saturn is unconscious. It should be pretty hard to keep them together for long.
Gemini's Mutable quality can really make them change from day to day if they feel pressured by Capricorn's rules and expectations. Gemini belongs to Air and as such don't feel the need to be down to earth as Capricorn does in its Earth element.
The main issue here would be their expiration date, since they are both pretty cold and distant, each one of them in their own way. Gemini would wait for the opportunity to get out of the boring Capricorn world, and Capricorn would probably calculate the recources spent while dating Gemini and how to stop the consumption.
Gemini likes to explore, leave all doors of possibility open and stay in the rational mind for as long as possible. Capricorn is carrier oriented, stable and has a ten year plan for the future. There is not much emotion in these descriptions as you can see. This doesn't mean they are not emotional, but when they come together, there is rarely someone there to light the spark and give the emotional flow to their relationship
Do you wonder how to conquer his/her heart? Get tips about Capricorn here.