Mercury runs this sign, and Mercury does not sit still
Gemini belongs to Mercury, the planet of exchange, wording and the speed at which a mind changes. That is the actual subject of any Gemini comparison: not whether two people like each other but whether they can keep talking without one of them tiring of it.
Mercury never separates from the sun by more than twenty-eight degrees, which means a Gemini sun has Mercury in Taurus, Gemini or Cancer and nowhere else. Those three make three quite different speakers. Mercury in Taurus is slow and will not be hurried; Mercury in Cancer talks in impressions and remembers tone rather than content. The shortlist notices none of this.
The four names and the angles behind them
Libra and Aquarius are the other air signs, each standing a third of the way round the wheel from Gemini. Leo and Aries are fire, each a sixth of the way off. Classical practice treats those two separations as the comfortable ones, and every sun-sign compatibility list in circulation is a translation of that fact into sign names.
What follows is a chain of reasoning with nothing at the end of it. The 120-degree separation was labelled easy; Libra sits there; therefore Libra suits Gemini. No stage of that involves a Gemini or a Libra.
What mutability actually contributes
Gemini is mutable, meaning it belongs to the end of a season and is built to adjust rather than to hold. Paired with fixed Aquarius or cardinal Libra, the traditional reading is that Gemini supplies motion and the partner supplies a direction to move in. That is a plausible description of a working arrangement and it is not a promise about anybody.
It also cuts the other way. Two mutable people can adapt to each other indefinitely and never settle anything, which is a real Gemini failure mode that no compatibility list warns about because the signs involved are not at the flagged angles.
The comparison that is worth doing
One sun sign out of two whole charts is a thin basis for anything. A comparison with substance reads both Moons for private need, both Venus placements for how fondness is expressed, both Mars placements for the shape of an argument, and both rising signs for first impressions, then measures the aspects that run between the charts. That measurement is synastry, and the synastry calculator performs it from two birth times.
Even done properly, it produces descriptions and not verdicts. The word soulmate has no astrological referent at all, and Gemini is unusually good at making a strong first conversation sound like one.
Better than checking a list
Take whichever sign is actually on your mind and read that combination directly. The pair pages describe what works, what grates and what the two modalities do to each other, none of which fits into a ranking.
With both birth times in hand the charts can be compared properly, and that is the only version of the question with a defensible answer.
Questions people ask about this
Which signs suit Gemini according to tradition?
Libra and Aquarius in air, Leo and Aries in fire. The grouping follows from the angles between them rather than from observation.
Is there a soulmate indicator in a chart?
No. Astrology contains no placement or aspect for the idea, and the term entered from outside the subject.
Why does Mercury matter for a Gemini reading?
Mercury rules Gemini and falls in Taurus, Gemini or Cancer for a Gemini sun, which changes how the person thinks and talks.
What should be compared instead of two suns?
Both Moons, Venus, Mars and ascendants, and the aspects running from one chart to the other.