Gemini and Sagittarius get along well because they're both changeable and lighthearted. There's also the spirit of the eternal student of life here.
They're quick-witted, bright spirits on a quest to absorb all the colorful things life has to offer. Gemini is a gatherer of ideas and Sagittarius a seeker of truth through experience. Together they move at a fast pace, enjoying adventures and letting life alter them as they go. The physical Sadge might be into more vigorous pursuits than Gemini, but both bring a flexible attitude to the bedroom. It's a promising match because every day will be different, keeping the relationship fresh and very alive.
In the Zodiac, these two are on opposite sides, and they definitely attract. They'll give each other plenty of space, and not try to cling too tightly. Neither will try to lock the relationship into stale habits, or push for a premature emotional commitment. Their loving style is to hold on loosely, and they can be philosophical when and if it's time to let go. They may even separate a few times, and return to the relationship renewed.
The main difference is direction, since Sagittarius aims its actions toward the future, and usually has a sense of forward motion. Gemini's path of growth is non-linear, which Sadge might interpret as scattered, manic and chaotic.
Sadge can help Gemini sharpen its focus and distill what it collects into wisdom. Gemini keeps Sadge from becoming dogmatic, always showing them new sides to a story. The Gemini-Sagittarius couple are friends first, and this can help them through the rough, non-romantic times. It's got the potential to be an enduring match, with new fascinations around every corner.
Read about the Dark Side of Gemini.