The fire sign of Aries, which is controlled by Mars, feeds the air energy of Gemini and causes things to spark. Aries are perfect partner
Jupiter rules Sagittarius, therefore they significantly deepen whatever relationship they are a part of. Sagittarius is also perfect.
Given that they are both air signs, Aquarius and Gemini are perhaps the most comparable. both understand each other
Leos are excellent Gemini soulmates because their ferocious energy infuses the union with enthusiasm.
Although it may come as a shock, Cancers make excellent soulmates for Gemini precisely because of how peculiar their bond is.