Gemini belongs to the element of Air and moves through life in the Mutable mode, ruled by Mercury. Sagittarius belongs to the element of Fire and moves in the Mutable mode, ruled by Jupiter.
Fire and air are natural allies — air fans fire, fire gives air direction. Together you think aloud, act boldly and rarely get bored. The balance to keep is between decisiveness and reflection.
Both of you adapt. The upside is flexibility and easy play; the downside is that important decisions can drift if neither of you plants the stake. Sometimes it helps to hold ground on purpose.
Both signs belong to the active, outward polarity, so your life together tends to be social, expressive and forward-moving. Give each other permission to pause sometimes.
What follows is a general read on this pair. Real compatibility depends on the whole natal chart — use the calculator linked on this page to compare two full charts when you're ready.