The Gemini is a jack of all trades, which means that dedicated aircraft do a better job at specific roles:
Before being upgraded, as a fighter it is on equal footing with the Wasp for air to air firepower and speed (with a slight health advantage), the Gemini costs more to build and takes longer to do so however, ultimately giving the advantage to the wasp due to UEF's ability to mass produce it using major experimentals.
As a bomber it falls behind the Eagle Eye in AoE. The Eagle Eye is also considerably cheaper to produce. The Gemini does have slightly more health, higher DPS, stronger shields and is a lot faster.
This unit offers the benefit of being a multi-tasker, meaning less micro management is required than if you had an air force of half fighters, and half bombers. It also allows you to adapt to changing threats much more smoothly.
Unlike the more dedicated Renegade, the Gemini features a modest radar system and double the visual range, making it useful as an early game scout.
Before research is taken into account, the Gemini is functionally identical to the Illuminate Weedoboth, save for faster shield regeneration of the Gemini. With research the Weedoboth will gain flares to redirect anti-aircraft fire, more health and a huge boost to it's anti-ground ability, while Gemini gains a considerable boost to it's anti-air capability and a good buff to speed.